THE 
 
 AMERICAN JOURNALOFSCIENCE 
 
 [FOURTH SERIES.] 
 
 
 
 Art. YII. — On an Artificial Lava-Flow' and its Spher- 
 ulitic Crystallization • by L. V. Pirsson. (With Plate I.) 
 
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 The material which forms the basis of the following article, 
 was obtained a number of years ago by the late Prof. Chas. E. 
 Beecher, of Yale University, at Kane, an industrial town with 
 glass-works, in McKean county, Penn., and presented to the 
 writer. It appears that one of the furnaces in the glass-works 
 accidentally broke, allowing the molten glass to escape; it 
 flowed into the pit below and cooled there. When cold the 
 mass of glass was blasted out and the broken material piled on 
 one side. It was from this broken material that the specimens 
 were obtained. The furnace is stated to have been 60 feet 
 long by 25 feet wide, and the molten glass in it at the time to 
 have been from 4 to 5 feet deep. This would have made 
 about 6000 cubic feet of glass, or say 700,000 pounds, an 
 amount which formed a flow of no mean size, when viewed 
 from the experimental standpoint. During the flow and 
 while cooling it partially crystallized, assuming features which 
 it is the object of this paper to present. An accident of this 
 kind to a glass furnace by which a flow of molten glass was 
 produced, which partly crystallized, has been described by the 
 late Professor Fouque.* In this case portions of the glass were 
 filled with milky white nodules with a greenish cast of color, 
 which were in places the size of a nut. These nodules were 
 spherulites which Fouque proved to consist of radially fibrous 
 prisms of wollastonite. Thanks to the kindness of Professor 
 A. Lacroix of the Musee d’histoire naturelle in Paris the writer 
 has been able to examine some of the glass described by 
 Fouque and to confirm his conclusions. The present material, 
 while resembling it in some respects, differs in several impor¬ 
 tant particulars, as will be presently shown. 
 
 * 
 
 Compt. Rendus, cix, Jan. 1, 1889. 
 
 Am. Jour. Sci.—Fourth Series, Yol. XXX, No. 176. —August, 1910. 
 7 
 
98 
 
 L. V. Pirsson—Artificial Lava-Flow 
 
 Morozewicz* lias also described crystallizations resulting 
 from the outflowing of glass from industrial furnaces; in this 
 case large single crystals of wollastonite were formed. He 
 mentions also rounded aggregates of fibrous diopside, but does 
 not describe them definitely as of spherulitic structure. 
 
 Spherulites .—The most important kind of crystallization 
 shown by the Kane glass-flow is in the formation of spherulites. 
 The glass itself is an ordinary green bottle glass, pale greenish 
 where thin and a clear, sea-green when a couple of inches thick 
 or more. This is more or less filled with white spherical 
 bodies which vary in size from that of very fine shot up to 
 those which are nearly the size of an egg. These, and espe¬ 
 cially the largest ones, may be readily broken out of the glass, 
 either as single spheroids, or as grouped or botryoidal masses. 
 When extracted they have a smooth, thin, outer shell of glass 
 which covers them like a skin. These solid bodies when 
 broken open are white, with a pale greenish tinge, and are 
 seen to possess a fibrous radiated structure. The fibers are 
 extremely fine and thread-like, giving to the surface the luster 
 of floss silk. This is true even in the largest. In some the 
 cross section of the broken spherule, as it lies in the glass, 
 appears radial but uniform, while in others changes in the 
 conditions of crystallization, as the fibers grew outwardly from 
 the center, have produced a series of concentric rings, such as 
 are sometimes observed in the smaller spherulites in rhyolite. 
 As many as ten of these rings, of almost perfectly circular form 
 and of similar width, have been observed in one spherulite, 
 and, thus mottling the silky sheen of the surface of the section 
 of the divided spherulite, they have a moire, or concentric, 
 watered-silk appearance, and add greatly to its beauty. The 
 structure, or consistency, of the spherulites varies greatly; 
 some are so compact that they are composed almost wholly of 
 crystalline material, while in others the thread-like fibers are 
 relatively widely separated and the whole spherulite is satura¬ 
 ted with the greenish glass. Thus, while the first form very 
 solid white objects, the latter appear almost like misty or cloudy 
 forms in the green matrix surrounding them, and the con- 
 choidal fracture passes through the glass without reference 
 to them nor can they of course he broken out and separated 
 from it. 
 
 In another portion of the flow which formed a sheet about 
 two inches thick the spherulites have a somewhat different 
 character. Here they appear to be composed, not of tenuous 
 fibers, but of distinct, rather thick, blades, 0*5-l*0 mm broad by 
 4 mm long, as may be seen by reference to figs. B and C in 
 Plate I which represents them in very nearly natural size. 
 
 *Min. Petr. Mitt., xviii, p. 124, 1898. 
 
and its Spherulitic Crystallization. 
 
 99 
 
 They average about 7-8 mm in diameter. In addition to the 
 regular spherulites various modifications of them exist in this 
 place, as may be seen by reference to the plate. Some are 
 composed of only a few blades, or even only two or three, 
 radiating from a center in a star-like group, but with blades 
 the same size and length as those of the more complete spher¬ 
 ulites. The beauty of these white radiate and stellate crystal¬ 
 lizations suspended in the clear sea-green glass is very striking, 
 and the fact that they are thus suspended and did not fall to 
 the bottom shows that at the time of their formation the molten 
 glass was in too viscous a condition to permit of such move¬ 
 ment, a fact whose bearing upon the question of their origin 
 will be treated later. The bottom of this sheet of glass is 
 composed of a layer of spherulites about 3-4 mm in thickness. 
 Looking at the bottom surface itself it appears flat and smooth 
 but so thickly covered with the radiate crystallizations of 
 these spherulites, seen in half section, that they nearly every¬ 
 where coalesce, or are contiguous.' The appearance recalls 
 surfaces covered with radiate zeolites or frosted window-panes 
 in winter. The upper surface of this layer presents a some¬ 
 what mossy appearance as the rounded, bladed, surfaces of the 
 spherulites project into the glass. It is seen on the right hand 
 side of figs. B and C in the plate. The white cloudy area near 
 the top of the piece in tig. C is due to the internal reflection 
 of light in the glass and nearly conceals a very fine spherulite. 
 The blades composing these spherulites are small at the center 
 where they unite and grow larger gradually as they extend 
 into the glass. They are four sided with apparent right angles 
 and are terminated by an oblique plane, but the crystallization 
 is rough and imperfect. Close inspection of the photograph 
 will show these details. 
 
 Mineral Composition .—The study of the mineral compos¬ 
 ing these spherulites in thin sections and in powdered grains 
 under the microscope proves that it is artificial diopside. This 
 is seen from the following properties: inclined extinction, 
 c on o measured to a maximum of 39°; sections having par¬ 
 allel extinction show the exit of an optic axis on the edge of 
 the field and that the plane of the optic axes lies in the clino- 
 pinacoid parallel to the length of the prism; by repeatedly 
 measuring the width of the nearly square prisms as they lie in the 
 glass their thickness is also obtained, and the birefringent color 
 yielded by those having the maximum angle of extinction indi¬ 
 cates that the maximum birefringence is about 0’03. The cross 
 section of the prisms shows them to be nearly square, but they 
 are too minute to show the cleavage parallel to the prismatic 
 sides ; the extinction bisects the angle. These are the proper¬ 
 ties of diopside and the physical determination was confirmed 
 
100 
 
 L. V. Pirsson—Artificial Lava-Flow 
 
 by qualitative chemical tests on material extracted from the 
 glass which proved, in addition to the silica, the presence of 
 traces of iron and alumina and abundant lime and magnesia. 
 It is evident that a dolomite limestone was used in the making 
 of the glass. 
 
 Microstructure of Crystals .—When studied under the 
 microscope in powdered form it is found that the crystal 
 blades, illustrated in the photographic plate, are very far from 
 being the solid continuous crystals they appear. Close inspec¬ 
 tion of them with a simple lens, as they lie in the glass, proves 
 them to have a parallel fibrous structure. The microscope 
 
 Figs. 1, 2. Skeleton crystals of diopside. 
 
 shows them in length sections to consist of bundles of 
 extremely slender rods or fibers. These are sometimes closely 
 packed, sometimes separated by much more than their own 
 diameters. They are surrounded by, or are cemented together, 
 by the glass in which they lie. Sections across these fibrous 
 bundles, or blades, prove that to a great degree they are not 
 simple solid fibers, or rods, but are more or less hollow, or 
 skeleton-like in form, and that groups of them have a similar 
 crystallographic orientation. One of these is shown in the 
 adjoining fig. 1. The directions of extinction in this are 
 indicated by the broken arrow lines; these indicate the plane 
 of symmetry in the compound or skeleton crystal and it is 
 interesting that the directions of growth are along both pina- 
 coidal and prismatic faces. Another type is shown in fig. 2 ; 
 the growth here is along the clinopinacoid l) : 010, and the 
 prismatic faces; that along the othopinacoid being wanting. 
 A number of different patterns of growth were observed but 
 they are sufficiently illustrated by these examples. 
 
and its Spheriditic Crystallization. 
 
 101 
 
 Cham Spherulites .—An interesting feature is the presence 
 of chain spherulites along flowage lines, as illustrated in the 
 photographic plate fig. A. The white lines are composed of 
 layers of innumerable minute spherulites, while the darker 
 layers between are of clear green glass free from crystalline 
 products. Examination with a lens reveals the fact that 
 almost without exception these lines are composed of a chain, 
 or layer, of single spherulites. Generally they are so closely 
 crowded that they coalesce to a considerable extent, less often 
 they touch at the circumference, and in some places they 
 appear quite regularly spaced but separated by areas of glass, 
 generally not wider than their own diameter. Viewed with 
 the lens from above, a layer appears like one of minute white 
 pills sifted into the glass, innumerable in number and spread 
 without order, here thickly clustered, there more thinly distrib¬ 
 uted. The average size of these spherulites is 0'5-O6 mm and 
 they do not vary much from it. Under the microscope in thin 
 section they appear like the spherulites of feldspar seen in acid 
 lavas, so much so that it is almost difficult to believe one is not 
 dealing with a section of rhyolite. Although snow-white in 
 the glass, and also as seen by the microscope with reflected 
 light from the section, they appear of a light leather-brown 
 color by transmitted light. With a low magnification the cen¬ 
 tral part of the spherulite looks uniform and homogeneous, the 
 fibrous character appears more evident as the outer boundary 
 is approached, and the outermost zone is composed of distinct, 
 branching, fibrous rays. With high magnification the central 
 part is also resolved into masses of excessively fine, tenuous, 
 matted fibers. At the center, where these fibers are cut per¬ 
 pendicularly, they appear as mere points. 
 
 When the structure of these bodies is revealed by the use of 
 crossed nicols, it is seen that they are not formed uniformly 
 of single fibers radiating from a common center, but rather 
 of groups, or bundles, of fibers somewhat radially divergent, 
 like brooms, or brushes; closely stellate groupings of which 
 form the spherulites. Thus they repeat on a minute scale the 
 examples shown in B and C of the plate and described in the 
 preceding section. Like other spherulites they show a black 
 cross, which is sometimes in one bundle, or brush, parallel with 
 the planes-of the nicols and sometimes, as the spherulite is 
 revolved and a new brush comes into position, is somewhat 
 inclined, proving that in each brush the majority of the 
 minute fibers tend to have orientated positions and thus make 
 a compound skeleton crystal. This is also like the larger ones 
 described and figured in the preceding section. 
 
 The light leather-brown color of the sections of these spher¬ 
 ulites is a curious feature. It is strongest in the central areas 
 
102 
 
 L. V. Pirsson—Artificial Lava-Flow 
 
 where the fibers are finest and most closely packed; at the 
 periphery, where they are coarser and more individualized, it is 
 wanting. With a very high magnification of the center and 
 parallel light from below, it disappears. With low-powered 
 objectives it is present, whether the lower nicol is present or 
 not. The spherulites of feldspars in acid volcanic lavas often 
 show the same phenomenon of brown coloration, and so do the 
 closely packed aggregates of minute scales of kaolin in feldspar. 
 Since in all these cases the component mineral fibers, or scales, 
 are white, that is to say colorless, this effect cannot be due to 
 a pigment, but must be an optical phenomenon due to the 
 absorption of light. An explanation of this may be somewhat 
 as follows : Angular fibers or scales of a colorless mineral 
 lying in all positions in a medium of lower refractive index 
 may act as prisms to rays of light passing through them. The 
 rays of white light would be then decomposed into their pri¬ 
 mary colors, and the violet rays more inclined than those of the 
 red. On passing upward and striking inclined surfaces of new 
 fibers, or scales, the light in the violet end of the spectrum, on 
 account of its greater inclination than that towards the red 
 end, would tend in greater measure to suffer total reflection 
 and be absorbed, while the less inclined rays of the red and 
 yellow would pass through and, illuminating the dark areas of 
 total reflection, give the fibrous mass the brown tone. This 
 explanation is based essentially on the same principle as that 
 given by von Federov* for the pseudo-pleochroism which 
 he observed in minerals containing thin parallel laminations. 
 Only in this case there is no parallelism, the angular fibers lie 
 in all directions, hence there is no apparent pleochroism and 
 the mass appears of the same brown color in all positions and 
 with, or without, polarized light. 
 
 It may be that in this lies the explanation of the brown and 
 gray colors and pleochroic effects observed in spherulites in 
 certain volcanic glasses by Cole,f but a search through the lit¬ 
 erature has not yielded to me any discussion of the frequent 
 brown color of spherulites (or kaolin) though so commonly 
 observed. Probably it is usually ascribed, if noticed, to some 
 ferruginous pigment, to which in some cases it is undoubt¬ 
 edly due. 
 
 Some other features of interest which these spherulites show 
 are as follows : In some, especially larger ones, hollow cavities 
 appear; others have no cavities but a pronounced crack run¬ 
 ning through the center ; often this crack passes along through 
 several of them, and it is always rigorously parallel to the 
 direction which the chain or line of spherulites makes (line of 
 
 * Min. Petr. Mitt., xiv, p. 569, 1895. 
 
 f Quar. Jour. Geol. Soc., xliv, p. 302, 1888. 
 
and its Spherulitic Crystallization. 
 
 103 
 
 flow). Where one has a cavity the ones immediately adjacent 
 are apt to have the crack. In some places this crack has 
 opened quite widely and an ellipsoidal or lenticular cavity is 
 produced, bordered on either side by half spherulites extend¬ 
 ing into the glass. The bearing of these facts is discussed in a 
 later place. 
 
 Another feature, quite similar to what is often seen in acid 
 volcanic glasses, is the presence of cracks in the glass immedi- 
 diately surrounding the spherulites. These commonly start 
 out from the edge of the spherulite, and especially from the 
 place where two tend to intersect, and then curve, sometimes 
 splitting and forming a Y. Usually after a short curve they 
 stop, but quite often they continue in a long curve concentric 
 to the outer surface of several adjacent spherulites and removed 
 from it a small fraction of the total diameter of one of them 
 (see fig. 3). It is owing to these concentric cracks surrounding 
 them that the larger spherulites may be readily broken out of 
 the glass and then appear with a thin, smooth skin, like var¬ 
 nish, coating them. 
 
 Fig. 3. 
 
 v 
 
 Fig. 3. Spherulites with cavities and cracks. 
 
 Stony Material .—In the material represented in the collec¬ 
 tion there are some pieces of a white lithoidal or porcelain-like 
 appearance. They have a light greenish tone of color and a 
 deeper green by transmitted light. A number of small ves¬ 
 icles or gas pores are here and there seen in the material. 
 Under the microscope it is found to consist of glass filled with 
 microlites ; some of rather short, minute prisms of diopside 
 with inclined extinctions and masses of feathery and fern-like 
 forms of the same substance. There are also minute fibers 
 whose extinction appears parallel and whose exact nature is 
 doubtful. 
 
 Obsidian. 
 
 One of the most interesting features of the collection is the 
 presence of several specimens of jet black artificial obsidian, 
 appearing quite similar in color, luster, and fracture to the 
 natural obsidians from the Yellowstone Park, Mono Lake, 
 Lipari Islands, and other well known localities. Flo wage lines 
 
104 
 
 L. V. Pirsson—Artificial Lava-Flow 
 
 or directions through the mass are indicated by parallel chains 
 of little points around which the glass has a minute but differ¬ 
 ent fracture and which thus interrupt the broad, smooth sur¬ 
 faces of the usual conchoidal areas. 
 
 The appearance of this black glass is so entirely unlike that 
 of the ordinary light-green and transparent bottle glass that 
 formed the flow that it is difficult to believe it is merely a mod¬ 
 ification of it, though positively so stated. It is, however, as 
 will be presently shown, a curious modification of a light-green 
 transparent bottle glass, though whether of the same melt as 
 that which has been previously described as containing the 
 splierulites seems doubtful. The doubt rests on chemical evi¬ 
 dence. The first glass contains, as mentioned, only a small 
 amount of iron, and a considerable proportion of magnesia, and 
 diopside has crystallized from it; the qualitative analysis of 
 the obsidian indicates much more iron, and while there is 
 abundance of lime, magnesia is present only in minutest traces 
 and as a result wollastonite has crystallized out in a particular 
 layer, as will be described later. Unfortunately, in the lapse 
 of time since the material was formed the possibility of obtain¬ 
 ing more exact information concerning it and the conditions 
 under which it was produced have been lost. If the statement 
 accompanying the collection is to be trusted, then magmatic 
 differentiation must have occurred, which seems hard to believe. 
 
 Considering the black color, it seemed at first as if the glass 
 must have been of some other melt into which some unusual 
 ingredient had entered which colored it black, or if a part of 
 the flow, then one which had in some way imbibed a coloring 
 constituent. Close inspection of a piece of this obsidian shows, 
 however, that in a place where there are fractures, or cracks, 
 penetrating it, if the mass is so turned to the light that the 
 rays entering it are reflected back from the internal surface of 
 the crack to the eye, the black color disappears and the glass, 
 between the crack and the eye, assumes its normal sea-green 
 transparent aspect. 
 
 The black color is due, then, not to a chemically diffused 
 coloring matter, so to speak, in the sense that iron compounds 
 color glass green, manganese purple, or cobalt blue, but is a 
 mechanical effect of some kind, owing to which that light 
 which strikes it and is not immediately reflected from the outer 
 surface, penetrates it and is absorbed. 
 
 Many or most natural obsidians which appear black are 
 found in thin section to be composed of a colorless glass, 
 swarming with specks, or trichites, whose exact nature is 
 unknown, but which many believe to be of magnetite. The 
 idea involved is this : An obsidian is formed because the effu¬ 
 sion and cooling have been 4 so rapid that the ordinary rock con- 
 
and its Spherulitic Crystallization. 
 
 105 
 
 stituents have had no opportunity to begin to crystallize before 
 the mass stiffened. But experience also shows that, in gen¬ 
 eral, the more rapid the cooling the greater the number of 
 centers of crystallization which will be set up. Now if any¬ 
 thing should start to crystallize in magma under such condi¬ 
 tions it would be the magnetite, ordinarily the earliest mineral 
 of importance to form, and it would be therefore distributed 
 through the glass in the shape of the finest dust acting mega- 
 scopically as a mechanical pigment and coloring it black. The 
 red color that many obsidians show may then be due to the 
 complete oxidation of the magnetite to ferric oxide. 
 
 On the other hand, a study of a number of sections of vari¬ 
 ous obsidians, and also of the literature, seems to indicate that 
 the color may not be wholly due to magnetite. Zirkel,* in dis¬ 
 cussing it, considers that it is sometimes inherent in the glass 
 (chemical so to speak) and is sometimes due to inclusions of 
 minute size, which he describes. In the latter case the glass 
 is colorless, and in the study of several obsidians of this char¬ 
 acter the writer has observed that the microlites, or trichites, 
 have a higher index of refraction than the surrounding glass, 
 as shown by Becke’s method. On lowering the objective 
 beyond the focal point they appear black, on raising it above 
 they become illuminated and appear colorless. Consequently 
 they are transparent and not magnetite. The greater the 
 number of these incipient crystallizations there are, the blacker 
 and less transparent the glass appears megascopically. The 
 black color in this case then is due to light absorption. In 
 one case where the slender microlites were arranged in parallel 
 positions in streams, it was noticed that the section possessed a 
 distinct pleochroism; when the ray vibrated across these 
 streams it appeared colorless ; when the long diameters were 
 parallel to the ray it was distinctly brown. This is an effect 
 of light absorption similar to that previously described under 
 spherulites and doubtless produced in the same way. The 
 black color of many obsidians then appears to be caused by 
 the dispersion, total reflection, and absorption of light due to 
 the presence of innumerable hosts of minute crystalline bodies 
 of a higher refractive index than the glass in which they lie, 
 such crystalline bodies being themselves colorless. 
 
 In the case of the artificial obsidian of Ivane, with magnifi¬ 
 cations of 540 diameters, the microscope reveals the presence 
 of bodies of indeterminate nature. They are so minute 
 that no definite shape can be assigned to them, but they 
 give a vague impression of being rudely octahedral. On lifting 
 the objective they become strongly illuminated, on lowering 
 they appear black. Thus they are transparent and of a higher 
 
 *Lelirb. d. Petrog., vol. ii, p. 271, 1894. 
 
106 
 
 L. V. Pirsson—Artificial Lava-Flow 
 
 index of refraction than the glass. While everywhere freely 
 sprinkled through the field of view, as the stars appear at 
 night'in the sky, they cannot be said to swarm in the sense 
 that, with reference to their own diameters, they closely 
 approach one another. They exert no perceptible action on 
 polarized light. To the absorption of light caused by the 
 presence of these minute bodies, as with some natural obsid¬ 
 ians, the black color of this variety of the Kane glass is 
 ascribed. 
 
 Partly Crystallized Obsidian .—In one place there is a 
 layer of the obsidian about 3 inches thick, which has partly 
 crystallized. The black glass described above makes a rather 
 clean and sharp contact with it. It is not now known whether 
 this layer was above or below the pure glass one, but as it con¬ 
 tains impurities on the side opposite the glass, and also on 
 general principles, it is assumed as the bottom part of the 
 obsidian. In the specimen it has a stony, not glassy, appear¬ 
 ance ; is of a very dark to blackish gray color, and is seen to 
 be composed of a sort of felt of innumerable small slender- 
 bladed crystals 2-4 mm long, which have a tendency to be 
 arranged parallel to the extension of the layer. It has some¬ 
 thing of a rough resemblance to some hornblende schists. 
 
 Fig. 4. 
 
 There is no schistosity in the fracture 
 however, which is rough and hackly. 
 The thin section under the microscope is 
 a very interesting one. It is composed 
 of a pale brown glass, filled with beau¬ 
 tiful crystallizations of the mineral 
 wollastonite. While in general the 
 mineral is developed as usual in long col¬ 
 umnar forms, or plates parallel to the 
 axis of symmetry, its growth has also 
 been in such aggregates of sheaf-like, 
 rosette-like, or feathery forms, that ac¬ 
 cording to the way these are cut differ¬ 
 ent effects are produced. The simplest 
 case is shown in fig. 4, which gives a 
 section parallel to b (010), across one of 
 the bladed crystals. The faces in the 
 zone of a (100) a c (001) are very 
 sharply developed, as may be seen from the following table 
 of angles measured against the cross-hairs : 
 
 Fig. 4. 
 section parallel to b (010). 
 
 
 
 
 Meas. 
 
 Calc. 
 
 a (100) 
 
 /'N 
 
 c (001) 
 
 84° 
 
 84° 
 
 35' 
 
 a' (100) 
 
 
 t (101) 
 
 50° 80' 
 
 50° 
 
 26' 
 
 a (100) 
 
 /\ 
 
 v (101) 
 
 42° 30' 
 
 44° 
 
 33' 
 
 a (100) 
 
 /\ 
 
 a (102) 
 
 70° 
 
 69° 
 
 55' 
 
and its Spherulitic Crystallization. 107 
 
 The calculated angles are those given by Grosser* and the 
 agreement is very good, except in v (101), which was not so 
 well developed as the other faces. The three cleavages paral¬ 
 lel to 100, 001, and 101 are excellent, as represented in the 
 figure, their excellence being in the order given. The plane 
 of the optic axes lies in the clinopinacoid and the bisectrix a 
 makes an angle of 33° in the acute angle /3 with the vertical 
 axis; Des Cloiseaux gives 32° 12'. The optical character is 
 negative, a being the acute bisectrix. Since, according to 
 Des Cloiseaux, the optic angle in air 2 E = about 70°, it follows 
 that one optic axis emerges almost perpendicular to c, 001, the 
 other at an angle of about 16° to a (100). Since both of these 
 are good cleavages, it follows that when one examines the 
 powder made by crushing the material, in convergent light 
 under the microscope with crossed nicols, almost invariably 
 each fragment exhibits the locus of an optic axis either just in, 
 or just off, the field of view. Since the a (100) cleavage is 
 the best, it is mostly the latter case that obtains and in the 
 blades the long direction is the one of least elasticity. 
 
 While these crystallographic and optical properties prove 
 the nature of the mineral, its identity was confirmed by the 
 fact that, when powdered, it dissolves readily in hydrochloric 
 acid and yields gelatinous silica. 
 
 It will be noted in the drawing of the crystal shown in fig. 
 4 that the a (100) faces are extended in a thin plate forming 
 re-entrant angles; commonly these thin plates extend far 
 beyond the main crystal and from each of the four corners, 
 tapering off indefinitely; the cross section of the whole then 
 shows an H with the vertical legs greatly extended. There 
 may be other cross connections producing ladder-like affairs. 
 It is also to be recalled that these extend as sheets perpendicu¬ 
 lar to the plane of the drawing, or along the b axis. More¬ 
 over the sheets are often curved and numbers of these extended 
 plates are grouped into sheaves or rosette-like groups, and thus 
 a variety of patterns are produced as these are cut by the sec¬ 
 tion at various angles. The individual filaments as they 
 appear in the section are commonly curved ; if examined with 
 a very high power it is seen that the curve really consists of a 
 series of short minute straight pieces of crystal with wedge- 
 shaped cracks between, continuity obtaining only along one 
 edge. By this curving, and by repeated branching, arborescent, 
 or plumose, forms are produced, and in places the glass 
 between crossed nicols appears filled with them and seems like 
 masses of magnificent ostrich plumes thickly scattered, the 
 beauty of whose effect is greatly heightened by the use of the 
 sensitive tint, which turns them brilliant blue and yellow. 
 
 * Zeitschr. f. Kryst., xix, p. 608. 
 
108 
 
 L. V. Pirsson—Artificial Lava-Flow 
 
 These plumose forms, which are of remarkable delicacy and 
 perfection, resemble the growths of augite in basaltic glass 
 from Hawaii, described by E. S. Dana*; they differ from the 
 growths in the pitchstone of Arran in being much larger, more 
 perfect, and in the curved or curled character of the stems, 
 thus resembling plumes rather than ferns. They never form 
 complete spherulites, though in places the thickly-grouped, 
 feather-like bunches partly resemble them. 
 
 Spherulitic Crystallization . 
 
 The vast majority of the natural spherulites occur in acid, 
 that is to say, siliceous volcanic glasses, and are composed of 
 quartz, or feldspars, or of these two minerals in various pro¬ 
 portions. The reason for this is because it is especially in 
 magmas of this nature that the relation between viscosity of 
 magma and crystal growth, which is necessary for spherulitic 
 crystallization and which is discussed later, is apt to occur. 
 Spherulite crystallizations may occur in basic magmas and are 
 known in the rocks called variolites,f but so far as the writer 
 has been able to discover, no discussion of spherulites, as such, 
 has been made which was not primarily based on material of 
 the kind referred to. It is natural, therefore, that in these 
 discussions the chemical character of the magma involved and 
 the nature of the component minerals are largely held respon¬ 
 sible as determinant factors for spherulitic crystallization. 
 Since such natural magmas contain, as is well known, volatile 
 constituents, especially water vapor, a considerable role has 
 been ascribed to its agency in this connection. Thus Crossj: 
 ascribed the origin of the spherulites in a rhyolite studied by 
 him to the presence of a colloidal condition in the magma, due 
 to the antecedent formation of masses of opaline silica contain¬ 
 ing the elements of feldspar, which caused their formation 
 and globular form. Iddings§ also, in his earlier discussions of 
 spherulites, suggests that water vapor plays an important role 
 in rendering certain places in the magma less viscous and 
 therefore more susceptible to molecular movement and crystalli¬ 
 zation. This would explain the formation of crystalline spher¬ 
 ulites in some places, while the surrounding magma solidified 
 as glass although at the same temperature. He says : “ Hence 
 we may conclude that the influence of the absorbed water- 
 vapor is to render the molecular mobility of the molten magma 
 
 * This Journal, xxxvii, 441, 1889. 
 
 f Pirsson, Petrog. of Igneous Rocks of Little Belt Mts., 20th Ann. Rep. 
 U. S. Geol. Surv., Pt. Ill, p. 532, 1900. 
 
 X Constitution and Origin of Spherulites in Acid Eruptive Rocks, Bull. 
 Phil. Soc. Wash., xi, 411, 1896. 
 
 §Bull. Phil. Soc. Wash., vol. xi, p. 446, etc., 1891. 
 
and its Sjpherulitic Crystallization. 
 
 109 
 
 greater at a given temperature in proportion to the amount of 
 hydration, thus permitting the crystalline arrangement of the 
 molecules in places of greater hydration, while the surround¬ 
 ing less hydrated portions are becoming too viscous.” 
 
 In his latest discussion of spherulitic crystallization, Iddings* 
 again refers the conditions controlling it in acid volcanic 
 glasses to varying amounts of water vapor—as u probably 
 dependent on viscosity, as affected by the gas contents of a 
 magma.” In these discussions, however, the underlying idea 
 appears to be not so much an explanation of the assumption of 
 the spherulitic form, or habit of growth, as of the production 
 of local conditions which would favor crystallization and 
 permit the formation of one component rather than another, 
 for in spherulitic growths in the natural volcanic glasses one 
 must deal with quartz and feldspar. 
 
 Spherulitic crystallization in the ultimate analysis is a ques¬ 
 tion of crvstal form or rather habit. The essential thing in a 
 typical splierulite is that from a common center crystals grow 
 in all directions whose elongation is excessive as compared 
 with their breadth and thickness. They may be straight rods, 
 or branching rays, or blades, or assume arborescent shapes, all 
 of which occur in the Kane specimens, but always tending to 
 elongate forms, thickly crowded. The suggestion of Cross 
 tends to assume that the spherical shape or rather area was 
 defined before crystallization actually occurred and is thus an 
 explanation rather of the outward form than of the inward 
 structure. It seems highly probable that the degree of hydra¬ 
 tion in the natural acid glasses affecting the viscosity, as sug¬ 
 gested by the writers above, plays an important role in deter¬ 
 mining the conditions and places for spherulitic crystallization, 
 but in the Kane glass this agency was not present and the 
 complication of having two mineral substances to deal with is 
 also wanting. The question here is simply one of the condi¬ 
 tions which determined the assumption of a certain kind of 
 crystal habit, and the answer, in the writer’s opinion, is to be 
 found in the degree of viscosity which had been attained at 
 the time when the saturation of the solution with the diopside 
 molecule reached the crystallizing point. Iddingsf states that 
 the habits of crystals depend in a large degree on the viscosity 
 of the magma, long slender prisms and branching shapes being 
 commonly developed when it is very viscous, though he does 
 not explain why this is so. The writer offers this explanation 
 for the slender fibers in the spherulites of diopside in the Kane 
 glass. The pyroxene has a prismatic cleavage and is elongate 
 
 * Igneous Rocks, vol. i, pp. 231, 233, 1909. 
 
 f Igneous Rocks, vol. i, pp. 206, 216, 1909. 
 
110 
 
 L. V. Pirsson—Artificial Lava-Floic 
 
 in the direction of this cleavage. It is not singular in this 
 respect, for most minerals tend to'be elongate, or columnar, in 
 the direction of pronounced cleavages. In the direction normal 
 to the cleavage faces the molecular network has a lesser 
 amount of cohesive attraction than in other directions and 
 hence the cleavage. We may therefore imagine that during 
 the process of growth the amount of molecular tension or pull 
 exerted by the crystal upon the unorientated molecules in the 
 magma is greater toward the end face than toward the prism, 
 or cleavage, faces. Hence the supply of molecules upon it is 
 niore rapid and the crystal grows faster in this direction. If 
 now the viscosity of the magma rises to such a degree that the 
 tension toward the side faces is not sufficient to overcome i^ 
 and orientate the molecules while that upon the end face is 
 sufficiently gleat, then the crystal will extend itself like a long 
 rod, or fiber, until the growing viscosity puts a stop to further 
 progress in this direction also. The crystallizing effect of the 
 fiber end would also be aided by the fact, that as the molecules 
 fall into position in the geometric network, or change from the 
 liquid to the solid state, a certain amount of heat is liberated, 
 tending to increase the mobility of the still unfixed adjacent 
 molecules and to render them more susceptible to crystallo¬ 
 graphic orientation. The crystals then, like a wire with a hot 
 end, bore out into the stiffening magma in all directions, and as 
 conditions are uniform about them they cease simultaneously 
 and the globular shape results. This relation of habit between 
 viscosity and cohesive attraction is also well illustrated by the 
 wollastonite crystals in the dark glass. They extend indefi¬ 
 nitely along the b axis perpendicular to which there is no 
 cleavage and are also tabular to the a (100) face, parallel to 
 which the best cleavage occurs. In the fibers of feldspar in 
 splierulites in the volcanic rocks the elongation in the major¬ 
 ity of cases is parallel to the clino axis, the direction of the 
 two prominent cleavages, The instances cited where the elon¬ 
 gation is parallel to the c axis* would tend to show that the 
 molecular attraction to the c (001) face’in orthoclase is greater 
 than that to the b (010) face. The fact that large crystals and 
 especially Carlsbad twins are often tabular or elongate on the c 
 axis also shows this. It is generally considered that the cleav¬ 
 age c(001) of orthoclase is better than that of b (010), and this 
 would appear to make these cases an exception to the general 
 rule. It is not clear, however, judging from the views of the 
 writers previously stated, that viscosity is the only factor to 
 consider in these cases, as it is in the Kane glass, as a delicate 
 balance between several different factors may have induced 
 
 * Iddings, Spherulitic Crystallization, Bull. Phil. Soc. Wash., vol. xi, p. 
 456, 1891. 
 
and its Spherulitic Crystallization . 
 
 Ill 
 
 this particular form. Nor does the writer wish to affirm as a 
 positive rule that in all minerals the attraction causing growth 
 is less towards a pronounced face of cleavage than in other 
 directions. It does, however, appear to be a rather general 
 
 one. 
 
 In addition to the elongation of the fibers in spherulites 
 there is also the branching to be taken into account. The 
 more rapid growth of the corners and edges of crystals, due 
 to their commanding a larger portion of the space which is 
 supplying the crystallizing material and thus producing branch¬ 
 ing and skeleton growths, was brought to attention by Leh¬ 
 mann* and further elaborated .by Bosenbuschf and needs no 
 further discussion here. In the treatment of the subject by 
 these authors, however, it is tacitly assumed that the molec¬ 
 ular attraction towards all faces of the growing crystal is the 
 same, or at least there is not mentioned anything to the con¬ 
 trary, and the view previously expressed by the writer, that 
 this may be different on different faces, brings into play another 
 factor. 
 
 That the molten glass had attained a considerable degree of 
 viscosity before crystallization began is clearly indicated bv 
 the spherulites seen in figs. B and C of the plate, since it was 
 great enough to support these denser bodies and prevent them 
 from sinking to the bottom. 
 
 That the formation of the spherulites was a comparatively 
 rapid process, after crystallization once started, is plainly shown 
 by their occurrence in the manner figured in A of the plate. 
 They were not present, of course, in the molten glass in the 
 furnace before breakage occurred, and their appearance here 
 in fiow lines proves they had formed before flowage motion of 
 the glass had ceased. However long a time viscous flowage 
 may continue in larger masses of lava, in this small body of 
 glass it must have had a relatively short period. 
 
 From the foregoing discussion it appears that the produc¬ 
 tion of spherulites in an anhydrous molten glass depends upon 
 crystallization starting from a center and proceeding out¬ 
 wardly in all directions at a time when the molten solution had 
 attained such a degree of viscosity as to control the crystal 
 habit and also upon the composition being such as to produce 
 minerals which naturally grow in columnar forms. The time 
 interval between this point and that where the fall of tem¬ 
 perature increases the viscosity to such a degree as to prevent 
 further molecular movement evidently controls the size of 
 the spherulites. In the case of the huge spherulites described 
 by Cross;); from Colorado this time interval must have been 
 relatively long. 
 
 * IJeber das Wachstlium der Krystalle, Zeitschr. f. Kryst., i, 462, 1877. 
 
 f Phys. der Min., 1885, p. 26, 4th ed., vol. 1, 361, 1904. % Loc. cit. 
 
112 
 
 L. V. Pirsson—Artificial Lava-Flow 
 
 In a sheet of molten glass the planes of cooling descend 
 vertically into the mass so that variations of temperature from 
 point to point become much more marked in this direction 
 than in a horizontal one. It is also true that in the change 
 from the liquid to the solid condition a more or less consider¬ 
 able contraction of volume ensues. Further, the force of 
 crystallization is very powerful within the distance in which 
 it acts,* and in the final stage of viscosity before the capa¬ 
 bility of molecular movement ceases the tension on the 
 
 c/ 
 
 growing crystal faces must be very strong and , per contra , on 
 the adjacent areas of unorientated molecules. Taking these 
 facts into consideration, it is clear that especially toward the 
 end of the process of spherulitic crystallization, these bodies 
 and the glass surrounding them will be subjected to stresses 
 which are most marked in vertical directions. Unable to with¬ 
 stand the tension, they may rupture, giving rise to horizontal, 
 or longitudinal, cracks or even be pulled apart so that ovoid, 
 or spherical, cavities are opened within them, as illustrated in 
 fig. 3. Or the tension between them and the surrounding 
 glass may be relieved by tangential, or radial, cracks in the 
 latter, as also illustrated and described. Thus the layers of 
 spherulites spread through the glass in bands by flowage become 
 elements of inherent weakness and along them it splits into 
 plates and thus has a laminated structure. This phenomenon 
 is also noticed in natural rhyolite glasses, like those from Lipari 
 and the Yellowstone Park, which cleave readily along the 
 bands of spherulites. Iddings,f in discussing the laminated 
 nature of such rocks, attributes it to non-homogeneity in dif¬ 
 ferent parts of the magma, produced by variable amounts of 
 contained water vapors, which unlike portions by the spread¬ 
 ing out action of flowing lavas become distributed in thin 
 sheets. Hence arise layers of different degrees of consistency, 
 crystalline character, etc., which condition the banded struc¬ 
 ture and cause lamination. The Kane glass, however, shows 
 that the same lamination, though not perhaps in so high a 
 degree, can be formed in the cooling of an anhydrous magma 
 and that the water vapor, except where it causes layers of bub¬ 
 bles, as in pumice, must be an indirect rather than a direct 
 agent in producing lamination in that it promotes more favor¬ 
 able conditions for crystallization. 
 
 Differentiation .—In the clear glass containing relatively 
 few small spherulites there is a non-liomogeneous quality which 
 shows itself by drawn out streaked lines and layers, these 
 being nowise different in color or consistency, but having dif- 
 
 * Becker and Day, Linear Force of Growing Crystals, Wash. Acad. Sci. 
 Proc., vo]. vii, p. 283, 1905. 
 
 f This Journal, vol. xxxiii, p. 43, 1887 ; Igneous Rocks, vol. i, p. 243, 1909. 
 
and its Sjpherulitic Crystallization . 
 
 113 
 
 ferent refractive indices, and thus becoming visible by tlieir 
 action on light, in the same manner that heated air-currents 
 are visible above a source of heat, or differing currents of 
 salt laden solutions are seen in a liquid. These streaks are 
 small and line and are parallel to one another, and to the 
 chains of spherulites, in the direction of How. Since the 
 refractive index varies in the different layers there must be a 
 difference in composition to occasion it. The difficulty of 
 obtaining glass of uniform composition, through a tendency to 
 separate into unlike portions, is one well known to makers of 
 lenses and other users of optical glass and has been frequently 
 described. It needs no further mention here beyond the com¬ 
 ment that this case adds another to those which have been 
 cited by others as a proof that the instability of molten silicate 
 solutions furnishes a presumptive proof of the possibility of 
 magmatic differentiation on a larger scale. 
 
 If we accept the statement, which cannot unfortunately be 
 now verified, that the black obsidian is really a part of the 
 same glass flow, a more striking instance of differentiation is 
 shown in that the clear glass is rich in magnesia and poor in 
 iron oxide while in the obsidian the reverse is the case. This 
 difference explains very clearly why diopside crystallized out 
 in the one and wollastonite in the other. But such a move¬ 
 ment, provided we suppose the original molten solution to 
 have been homogeneous, whereby magnesia and ferrous oxide 
 are concentrated in opposite directions, while one would hesi¬ 
 tate to say it was impossible, from all our experience gained 
 in the study of rock-masses in which these oxides move together, 
 seems very unlikely to say the least. And on the other hand, 
 we do not know, in the production of glass on so large a scale, 
 whether a sufficiently high temperature was maintained long 
 enough to permit the molten solution to assume conditions of 
 uniformity throughout its mass, if, on account of a difference 
 of materials used in the making, uniformity was not present to 
 begin with. Viewed from this standpoint, it is possible that 
 the obsidian was part of the same flow. At all events, the 
 material seems to indicate the possibility of obtaining in this 
 direction, artificially, some light on the, at present, mysterious 
 process, or set of processes, known as differentiation. 
 
 Composition of Minerals .—The use of the terms diopside 
 and wollastonite in this paper is of course only a general one 
 and it is not assumed that these substances are necessarily 
 present in a pure condition. From the splendid results which 
 have been achieved in recent years by Dr. Day and his 
 co-workers in the Geophysical Laboratory of the Carnegie 
 Institution concerning the properties of the lime and magnesia 
 silicates, and the conditions under which they are formed, and 
 
 Am. Jour. Sci.—Fourth Series, Vol. XXX, No. 176. — August, 1910. 
 
 8 
 
114 
 
 L. V. Pirsson—Artificial Lava-Flow. 
 
 which have been published in this Journal,* it has been shown 
 so clearly that diopside and wollastonite, as produced from 
 molten solution, may contain variable amounts of lime and 
 magnesia metasilicates in solid solution, that it would seem 
 quite possible that the diopside in the Kane glass is far from 
 being pure in composition. The analytical test proves that 
 the wollastonite is relatively a pure compound, and the optical 
 data, so far as they go, agree with this view. 
 
 It is evident from the works quoted that the temperature of 
 the glass, when the wollastonite crystallized, must have been 
 lower than 1190°, since this is the inversion point of the 
 mineral to pseudo-wollastonite, which alone exists above that 
 temperature and up to 1512°, its melting point. The melt¬ 
 ing point of diopside is 1380° and the temperature of the 
 molten solution was therefore originally above this point; this 
 of course is to be expected, even though a flux is used to help 
 carry the quartz sand and lime carbonate into solution. 
 
 Summary .—-The study of the accidental flow of molten 
 bottle-glass at Kane, Penn., has, brought out the following 
 chief points: 
 
 That spheruiites of varying size and character and consist¬ 
 ing of diopside may be formed in an anhydrous molten solu¬ 
 tion by rapid cooling. 
 
 That the spherulitic type of crystallization appears to be 
 conditioned by the relation of crystal habit and properties to 
 the viscosity of the magma. The spheruiites are of rapid 
 growth. 
 
 That the brown color, which many spheruiites exhibit by 
 transmitted light, is a phenomenon of light absorption. 
 
 That obsidian may be artificially produced from a clear glass 
 and that its black color is a phenomenon of light absorption. 
 
 That artificial wollastonite may exhibit certain characters 
 which are described. 
 
 o 
 
 Sheffield Scientific School of Yale University, 
 
 New Haven, Conn., April, 1910. 
 
 *Day, Shepherd and Wright, The Lime-Silica Series of Minerals, vol. xxii, 
 pp. 265-302, 1906. Allen, White and Wright, On Wollastonite and Pseudo- 
 wollastonite, Polymorphic Forms of Calcium Metasilicate, vol. xxi, pp. 89- 
 108, 1906. Allen, White, Wright and Larsen, Diopside and its Relation 
 to Calcium and Magnesium Metasilicates, vol. xxvii, pp. 1-47, 1909. 
 
Am. Jour. Sci., Vol. XXX, 1910 
 
 Plate I 
 
 Spherulites in Artificial Glass