Reprinted from the Journal of Geology, Vol. XIV, No. 4, May-June, 1906 Return this bo ok on or before the CONDITIONS' OF FOSSILIZATION J. CULVER HARTZELL 1 PRINTED AT THE UNIVERSITY OF CHICAGO PRESS t ♦ 1 I b CONDITIONS OF FOSSILIZATION- 5&b J. CULVER HARTZELL University of the Pacific, San Jose, Cal. Introduction. Superficial Consideration of Fossils. Conditions in Which Fossils Occur. Difficulties Encountered. Authors’ Definitions of Molds and Casts. Definitions of Molds and Casts Followed. Definitions of “Original,” “Mold,” and “Cast” Proposed. Lithological Character of Formations, as Affecting the Preservation of Inverte¬ brates. General Character of Invertebrate Skeletons. General Mineral Character of Living Invertebrates. Replacing Minerals. Horizon, Locality, and Lithological Character of Formations Studied. Minerals Replacing Original Minerals Secreted by Invertebrates. INTRODUCTION In the study of a fossil the first important point to determine is its state of preservation—whether the fossil under consideration be the original , a cast of the original, or a mold of the original. By not observing this precaution, errors have been made, and will continue to be made unless it be definitely understood what is meant by these terms and the conditions they represent in fossils. SUPERFICIAL CONSIDERATION OF FOSSILS A superficial consideration of a fossil is often apt to lead to a misinterpretation of its condition of preservation, for fossils vary in this respect. Molds may be taken for casts , and described as exhibiting the external structure of the original. 1 This paper was largely prepared in 1898-99, under the direction of the late Professor Charles E. Beecher, when the writer was a graduate student at Yale, and was submitted for the degree of master of science. Its publication has been delayed because the writer wished to collect further data, which he has done in Europe and America. Another paper is in course of preparation, in which an attempt to formu¬ late laws governing conditions of fossilization will be made. The writer also wishes to take this opportunity to thank Professor S. L. Penfield, of Yale, for valuable suggestions during the preparation of the original manuscript. 269 f l32«l 270 J. CULVER HARTZELL We find the same organisms preserved in all manner of conditions, and only by a careful comparative study of the exterior and interim* markings are misinterpretations to be avoided. Fossils of the same species have been referred to different species, and the same genera to different genera. For example, Miche- linia clappii , Hall , 1 was misinterpreted by Edwards and Haime, and referred to Chonostegites clappii 2 3 4 * and also to Emmonsia (?) cylindrical and by Billings to Haimeophyl- lum ordinatum 4 and also to Michelinia inter mit¬ tens. 5 (See Figs. 1, 2, 3> 4, 5, 6 > 7-) Th is misinterpretation was due to the peculiar ways in which this genus occurs. At times the walls are coated with silica, and then Fig. 1 .—Chonostegites clappi. (After Edwards filled in with calcite. and Haime.) At times only the silice¬ ous coatings are left, which give a mold of the inner walls. Then again we find that they were filled with calcite, the walls having disappeared; and in this case we have a solid mold instead of a hollow one. At times the form is partly destroyed, leaving molds, casts, and parts of the original in the same specimen. We find fossils of the same species preserved (a) in their original condition, ( b ) as casts, and ( c ) as molds. In regard to the first condition little difficulty will present itself. The second and third, however, may lead to confusion, for they may not exhibit the exter¬ nal form. 1 Hall, Geology of the State of New York (1876), “Illustrations of Devonian Fossils, ” Plate XVII. 2 Edwards and Haime, Pal. Fos. d. Ten. Pal. (1851), p. 299, Plate XIV, Figs. 4, 4a. 3 Ibid. 4 Billings, Can. Jour. U. S., Vol. IV (1859), p. 139. s Ibid., p. 113. CONDITIONS OF FOSSILIZATION 271 CONDITIONS IN WHICH FOSSILS OCCUR The following illustrations will show the various conditions in which fossils are found, and they will also serve to show the necessity for close observation and comparative study. A. The original skeleton may be preserved. If there be hollows or spaces, they may become filled with infiltrating material. In a case of this kind little difficulty will present itself in the determina¬ tion of the fossil. B. The original skeleton may be replaced by some mineral and the cavities filled with the same, or some other material. If the skeleton were composed of aragonite, and were replaced by calcite, the external form and markings would be preserved; but the internal organic structure would be lost, and hence not seen under the microscope. In this case we should depend upon external markings for identification. If a skeleton composed of calcite were replaced by pyrite, we could not ascertain in thin section whether the internal organic structure were lost or preserved, because of the opaqueness of the pyrite, and again we should depend upon external markings. A broken section, however, will show on the fractured surface the minutest details, in many cases. If the organism were replaced molec- ularly by a mineral which transmits light, the internal organic structure would be so well preserved as to be readily distinguished under the microscope. In a case of this kind we should have a double check—the internal as well as the external structure—and its identification would be doubly sure. Fig. 3. —.Chonostegites clap pi. (After Miller.) Fig. 2. —Chonostegites clappi. (After Edwards and Haime.) ■ 272 J. CULVER HARTZELL C. The organism may disappear after its cavities or hollows were filled by infiltrating material. In this case we should have only the impression of the interior of the original, and it would be neces¬ sary to compare this with the interior of skeletons already known. This is at times difficult; but it is the only alternative. To make this comparison it might be necessary to take an impression, or cast, from the exterior of this filling, which cast would show the markings of the interior of the original. Unless we could find a fossil or a Fig. 4 .—Michelinia clappi. (After Hall.) living form showing the same, or nearly the same, internal markings, we should be baffled in any attempt to adjust it to its zoological position, and hence its identification would remain unsolved until one of these two conditions was satisfied. D. The hard parts of an organism may leave only an impression of its exterior in the matrix. It will then be necessary to take an impression, or cast, from this first impression, or mold, and upon the markings shown on this cast will depend the identification of the fossil. CONDITIONS OF FOSSILIZA TION 273 E. The exterior of the skeleton may become coated with some. mineral such as silica, after which the skeleton may disappear. In this case we should have a hollow mold. It would then be neces¬ sary to take an impression, or cast, from this mold in order to ascer¬ tain the external markings of the original, and by comparing this cast with known forms we can determine its identity. At times the corallites in a compound coral will become coated with silica, and the spaces between the corallites filled with calcareous material Fig. 5 .—Michelinia clap pi. (After Hall.) the entire corallum having disappeared, leaving a mass filled with these hollow siliceous tubes, the inner surfaces of which will be molds of the exteriors of the corallites. (See Michelinia , pre¬ viously referred to.) We may have the exterior coated with silica, the skeleton then disappearing, and the space left filled with calcite. In a case of this kind we have molds of the exterior, and it is impossible to identify the fossil without first dissolving out the calcite, after which the procedure will be as already described. 274 /. CULVER HARTZELL The interior cavities or hollows of a coral may become coated with silica, after which the skeleton may disappear, leaving molds of the interior which will have the appearance of a sponge. If we had a coating which had been deposited upon the inner surface of the shell, it might be easily determined by taking a cast for com¬ parison with other shells; but in the former case its identification would be extremely difficult, for we might not be able to secure casts from these molds. Even if this could be done, we might still have difficulty in its identification, for we should have only casts of the interior of the cavities or hollows of the original for com¬ parison. DIFFICULTIES ENCOUNTERED From the foregoing it is plain that difficulties present themselves even when it is known that the fossil in hand is the original , a mold, or a cast; but the difficulties increase if it be not known what the condition of preservation is. Cases present themselves in which it is a very difficult matter to decide whether the fossil is a cast or a mold; but in the majority of cases this difficulty is obviated by close observation and an understanding of the meaning of casts and molds . It is only by a study of casts and molds in their various condi¬ tions—found as fossils or made in the laboratory—that we may with a certain degree of exactness determine the condition of pres¬ ervation of a fossil. The internal markings of some forms resemble the external markings of other forms, and it is only in the above way that we may be certain that we are dealing with external or internal markings. There is a wide difference between a cast and a mold. Ca ts vary in that some do and some do not show the structure of the organ¬ ism. Receptaculites oweni, Hall, from the Galena (Lower Silu¬ rian) at McGregor, Iowa, represents the inner surface of the skele¬ ton, and is a cast. (See Figs, i and 8 .) “In most specimens . . . . the remains consist of the filling of the intermural space, with casts of the outer surface of the inner wall, the inner surface of the outer wall, and of the connecting tubes .” 1 This is a calcite 1 Bernard, Principles of Paleontology, Fourteenth Annual Report, New York Geology (1895), pp. 89, 90. CONDITIONS OF FOSSILIZATION 275 cast of the interior, and has been regarded as having the structure of the original organism. The conditions were evidently such as to preclude its preservation in its original condition, or in such a condition as to render its determination certain. authors’ definitions of molds and casts Authors differ in their definitions of molds and casts. Some make the terms synonymous. Others define them separately, but are not consistent in their application. Darwin 1 considers the mold as a matrix, and the impression made by an organism in this matrix he terms a “cast. ” Bernard 2 applies the term “mold” to three distinct results: ( a ) to an impression made by the exterior or the in¬ terior of a shell; (b) to molecularly replaced organisms; and (c) to fillings cf impressions. Gratacap 3 applies the term “cast” to (a) fillings which take the Fig. 6 .-MicheUnia clappi. (After Rominger.) place of organisms, to (b) the material filling the space occupied by the soft parts. The term “mould” he applies to impressions of the exterior of an organism. White 4 applies the term “mold” to impressions made by the organism. To the material filling this “mold” he applies the 1 Darwin, Geological Observations , Vol. II, p. 414. 2 Bernard, op. cit. 3 Gratacap, “Fossils and Fossilization, American Naturalist, Vols. XXX, XXXI, pp. 288, 902, 903. 4 White, Con. of Pres, of Inver. Fos., Bull. U. S. G. & G. S., Vol. V, No. 1, p. 135. 276 J. CULVER HARTZELL term “cast.” He 1 also uses the term “histometabasis” for the condition which produces a molecular replacement or substitution or paramorphism . 2 He uses the term “fossil pseudomorphs ” for the materials occupying cavities formerly occupied by shells, the occupation having taken place by precipitation due to infiltra¬ tion. He uses the term “fossil molds” for “cavities in sedimentary rocks which were originally occupied by fossils,” and says that “the original sur¬ face features and markings are often minutely preserved in molds.” He also uses the term “casts” for “counter-parts of fossils,” and also for the material which may occupy the animal chamber. He further speaks of making “artificial casts of natural molds” in order to get “the original form and surface features.” Geikie 3 applies the term “mold” to impressions made by the organism. Fig. 7.— Michelinia clappi. (After Rominger.) To the material filling this “mold” he applies the term “cast.” He 4 also applies the term “cast” to the material occupying the animal chamber. Von Zittel 5 applies the term “mold” to impressions. The term “cast” he applies to the material which occupies the “interior of a shell or hollow body.” 1 White, “Relation of Biological and Geological Investigations, Proceedings o) the U. S. N. M., Vol. XV, pp. 264-67. 2 Dana, Text-Book 0} Mineralogy, (1898), p. 293. 3 Geikie, Text-Book 0} Geology, 3d ed., p. 651. 4 Geikie, Outlines 0} Field-Geology, 5th ed., p. 78, Fig. 14. 5 Von Zittel, Text-Book 0} Palaeontology, Eastman translation, Vol. I, Part 1, p. 2. CONDITIONS OF FOSSILIZATION 2 77 Woods 1 applies the term “mold” to (a) the impression, to ( b ) the material filling the space occupied by the animal. The term “cast” he applies to ( a ) the material filling the space occupied by the organism, and to ( b ) the material filling the internal cavity or cavities. Nicholson and Lydekker 2 use the terms “meld” and “cast” interchangeably. Lyell 3 applies the term “mold” to the matrix in which an impres¬ sion of the exterior has been made. The term “cast” he applies Fig. 8.— Receptaculites oweni , Hall. (From specimens in Yale Museum.) to (a) the material filling the interior of the organism, and to (b) the material filling the space left by the organism. Penning 4 uses the term “unchanged fossils” for unaltered shells or valves, cr those which have lest only the animal matter. “Re¬ placed fessils” he applies to the material which has been substi¬ tuted for original material of the shell. “Internal cast” he uses for “the impression or reversed facsimile oj the external jorm of the organism 5 that once filled the empty space” of the shell. The term “external cast” he uses for impressions made by the exterior of the shell, and says that “by taking an artificial cast from the 1 Woods, Palaeontology , 2d ed., pp. 6, 7. 2 Nicholson and Lydekker, Manual of Palaeontology , 3d ed., Vol. I, pp. 5, 6. 3 Lyell, Students' Elements of Geology , 3d ed., pp. 42-46. 4 Penning, Text-Book of Field-Geology, 2d ed., pp. 208-12, and Fig. 29, p. 211. s The writer’s italics. 278 J. CULVER HARTZELL external impression” we obtain “an accurate representation of the pre-existing 'shell. ” Williams 1 is perplexing in his use of the terms “mold” and “cast.” He says: Thus a fossil .... may consist of the shell now removed, in which case it may be the reverse or cavity over the exterior of the shell, or ... . similar impressions of the inner surface; or the cavity may be again filled with detrital matter, forming a cast of either the inner or outer form of the shell or object fossilized; in the former case it would be called a mold; in the latter, a cast. Schuchert 2 applies the term “mold” to impressions of the exte¬ rior and speaks of the mold as “preserving the exterior form and ornamentation” of the shell. He is ambiguous in his use of the term “cast” for he may be referring to a matrix which contains concave impressions (impressions of the exterior of a valve) or i convex impressions (impressions of the interior of a valve), or to the material which replaces a valve. DEFINITIONS OF “MOLDS” AND “CASTS” FOLLOWED The definitions followed in this paper are the following: A mold is “a form or model pattern of a particular shape, used in determining the shape of something in a molten, plastic, or other¬ wise yielding, state.'” “In founding, a viold is the form into which a fused metal is run to obtain a cast. ” 3 The mold determines the shape of the material put in or upon it, and this material, when removed, will be an exact duplicate of the object from which the mold was made. This removable material is termed a cast. The depressions in the original object will appear as protuberances in the mold, and the protuberances as depressions. The cast will show the depressions and protuberances as they appear in the original. The surface of the original object upon or around which the mold is made may be either convex or concave. If it be concave, the mold will be convex, and vice versa. A mold with a convex surface is called by some authors a “cast.” If the skeletal part 1 Williams, Geological Biology (1895), p. 79. 2 Schuchert, “Directions for Collecting Fossils,” Part k, Bulletin No. 39, U. S. N. M., p. 13. 3 Century Dictionary , “Mold.” CONDITIONS OF F0SSIL1ZATI0N 279 of an organism be hollow or has a concavity, this space is considered a “mold” from which a “cast” is taken. This is obviously wrong, for in this case the “mold” is the object , and the “cast” the mold from which a cast may be taken, and this cast will be a duplicate of the object. Then, again, an impression from a convex object is termed by some a “cast.” This is also obviously wrong, for the “cast” would have the markings of the object in reverse order; hence it would be a mold. The general concavity or convexity of a surface will not deter¬ mine it as a mold or a cast. Such determinations depend upon the markings of that surface. DEFINITIONS OF “ORIGINAL,” “MOLD,” AND “CAST” PROPOSED.. In attempting to fix the meaning of the terms“original,” “mold,” and “cast, ” it is hoped the following definitions will prove acceptable, especially the latter two: I. The term original is used to designate an organism 1 that has not lost its original structure or composition, to any appreci¬ able extent, in the process of fossilization, except the organic matter which may have filled the interstitial spaces. (See Fig. 9 a and b.) II. The term mold is used to designate the imprint of the ex¬ terior or interior of an organism. Fig. 9.— (a and b) originals; (c)mold (a) If the organism leave an im- of interior. print of the exterior, this imprint is a mold oj the exterior. (See Fig. 10.) ( b ) If the hollow organism become filled with material this materia^ 1 Strictly speaking, a lifeless animal is not an organism; but in common parlance the lifeless body is an organism because it is that which at one time functioned. Like¬ wise, we speak of the products of life as organic. Therefore, for lack of a beter term, organism is used to denote the harder parts of animals which we term fossils, the softer or destructible parts of which have decayed and passed away. The term “organism” can in no sense, however, be applied to molds and casts, although these are fossils as much as the unaltered skeletal parts of animals. 28 o J. CULVER HARTZELL is a mold oj the interior. (See Figs, gc, n, 12.) (c) It follows from the above that, if the hollow organism become filled with and im¬ bedded in material of the same, or different, composition, and then disappears, we have left a mold of the exterior in the matrix and a mold of the interior in the form of a kernel. (See Figs, gc, 10.) III. The term cast is used to designate the material which takes the place of the original, whether by replace¬ ment due to a molecular process or to infiltration. It is also used to designate the material occupying the mold made by the exterior or interior. (See Figs. 10, 12, 13, 14.) If the mold described under II (a) becomes filled, we have a cast of the exterior. If the mold described under ( b ) becomes imbedded, its imprint will be a cast of the interior. If the space between the two molds described under (c) becomes filled, we have a cast of the exterior and interior, and therefore an object the same in shape and outline as the original. If the original be gradually replaced molecularly by some mineral, we have a cast which will show its shape, outline, and internal structure. From the foregoing it is obvious that an original is the organism itself; a mold, the reverse of the original; and a cast, the counter¬ part of the original. The latter may or may not show the internal organic structure. It follows, therefore, that the only way one may know whether the markings on certain molds or casts represent the exterior or CONDITIONS OF FOSSILIZATION 281 interior of known forms is, as has been previously said, to make a study of molds and casts, and thus reduce the liability of mistakes to a minimum. THE LITHOLOGICAL CHARACTER OF FORMATIONS AS AFFECTING THE PRESERVATION OF INVERTEBRATES The conversion of an organism into a fossil depends upon the character of its skeletal parts, the material in which it is buried, and the material brought in, in solution, by infiltration. The material of which the skeletal part is composed varies in different groups, being more durable in some than in others, and therefore plays an im¬ portant part in the preservation of the organism. The variation in the lithological character of the material in which the organ¬ ism is buried also plays an im¬ portant part in its preservation. Certain organisms are preserved as originals; others as molds and casts, in the same forma¬ tion and locality. In this same formation, but in a locality of different lithological character, those groups which were lost under the former condition may be retained under the latter, and vice versa. Apparently a law could be formulated to the effect that organisms of the same mineral composition will be preserved in the same man¬ ner, as originals, molds, or casts. In reality, however, this is not true. Organisms are more completely preserved as originals in limestone; yet it is in limestone that we find the most casts by molecu¬ lar replacement. Molds and casts are very common in sandstones. As limestone approaches dolomite, the molds and casts increase, although we also find originals. We find molds in hematite; but they are more rare than in sandstones. The most perfect fossils are found in sandy and clayey shales. The Niagara group at Waldron, Ind., is made up of fine calcareous shales which are overlain by limestone. In these shales we find Fig. 12.—Mold of the interior. 282 J. CULVER HARTZELL quantities of corals, bryozoans, and crinoids. The brachiopods are more or less preserved as originals. The sponges, gastropods, annelids, and crustaceans are well represented. The lamellibranchs and cephalopods, however, are absent. The Paleozoic hexactinellids occur in groups of strata containing other organisms; but in their own particular beds the absence of other forms is striking. In Steuben county, N. Y., the formation is a sandstone which is fine-grained and argillaceous, and contains very few crinoids and brachio¬ pods; but Hydnoceras tube¬ rosum occurs in abundance. 1 The fossils in the sandy and gravelly deposits of the Potsdam, the Medina, the Chemung, the Catskill, the coarse con¬ glomerates of the Lower Carboniferous, or the calcareous grit of the Schoharie and Oriskany, all show different conditions of preservation. The trilobites in the Potsdam of Wisconsin and Minnesota are badly broken up. In the Cambrian of Wisconsin we find argillaceous layers in which are molds and casts. The Medina sandstone contains poorly preserved fossils, and shows molds of the interior. In the calcareous sandstones of the Chemung we find well-preserved organisms; but in the Catskill sandstones they are poorly preserved. The Utica, Marcellus, and Genessee slates show well-preserved originals; but the majority of the fossils are molds. 1 Beecher, Memoirs 0} the Peabody Museum, Yale University, Vol. II, Part 1. CONDITIONS OF FOSSILIZATION 283 The articulate Brachiopoda, the Anthozoa, and the Bryozoa of the New York Hamilton Shales are. well preserved; but the Mollusca occur as molds and casts. Trilobites, inarticulate brachi¬ opods, and ostrocods are well preserved. In the Carboniferous shales of Illinois the Mollusca are well preserved. In the Mesozoic sandstones of the West the fossils are casts and molds generally, with the exception of the Ostrea and allied genera. The best fossils, as a rule, are found in those limestones which contain more or less argillaceous or siliceous ingredients, as in the Waldron beds of Indiana, the Hamilton layers of New York, and the Lower Carboniferous of Crawfordsville, Ind, The Schoharie grit, the Oriskany sandstone, and the Calciferous beds along Lake Champlain give siliceous molds on weathering. The Upper Helderberg limestone gives molds and casts better than sandy deposits in general. At Cumberland, Md., however, the brachiopods are perfectly preserved as casts of silica in the Oriskany sandstone. In the Galena limestone many of the fossils are preserved as casts composed of galena. The coal-measures show molds coated with pyrite. In the Clinton of Oneida County, N. Y., we find limonite casts. In the Trenton of Wisconsin and Tennessee we find casts of silica. In the Niagara limestone of western New York we find calcite casts; but on the weathered surfaces they are sili¬ ceous. In the Schoharie grit we find siliceous molds of the interior of brachiopods. In general, calcareous skeletal parts show an unequal persistence as fossils in their original condition. Chitinous skeletons are never preserved in their original condition. , GENERAL CHARACTER OF INVERTEBRATE SKELETONS c Chitin is confined to the Arthropoda and a few brachiopods which are made up of alternating layers of phosphate of lime and chitinous material, as in Lingula anatina and in the graptolites. Silica is confined to the arenaceous Foraminifera, the Radio- laria, the Silicispongia, and Diatoms. Calcareous material is confined to the porcellaneous and vitreous 284 J. CULVER HARTZELL TABLE I Horizon, Locality, and Lithological Character of Formations, and the Condition of Preservation of Classes, Studied Condi- Horizon Locality Lithological Character Class tion of Preser¬ vation Remarks Carboniferous Yellow Cr., Shale Brachiopoda 0 M* M exterior and interior * Ohio Subcarbon* Crawfords- Limestone Spongiae C Pyrite iferous ville, Ind. Mazon Cr., Limestone Arachnide M C Exterior Ill. Burlington, Limestone Anthozoa C Silica la. Crinoidea C Calcite, silica Brachiopoda O C C siliceous (Waverly) Warren, Pa. Sandstone Anthozoa C Exterior, sandstone Crinoidea M Heads and arms, sandstone Ophiuroidea M Exterior, sandstone Brachiopoda OMC M exterior and interior, sil¬ ica; exterior, sandstone; interior, sandstone, calcite. C exterior, silica Lamellibran- O Slightly changed chiata Gastropoda M Exterior, sandstone Cephalopoda M C Sandstone (Chester, Pulaski Co., Limestone Bryozoa O Slightly changed top of) Ky. Limestone (St. Louis Breckenridge Anthozoa C Silica Upper) Co., Ky. Limestone Brachiopoda O Slightly changed Mt. Vernon, Brachiopoda c Exterior, silica Rockcastle Gastropoda c Silica Co., Ky. (St. Louis) Taylor Co., Limestone Anthozoa c Silica Ky. Brachiopoda c Silica Scaffold Cone Limestone Anthozoa c Silica Ridge, Mad¬ ison Co., Ky. Brachiopoda c Silica (Keokuk) Russellville, Limestone Blastoidea c Silica Ky. Kings Mt., Limestone Anthozoa 0 Spaces filled with calcite Lincoln Co., Ky. Keokuk, la. Shale Anthozoa 0 Spaces filled with calcite Crinoidea c Silica Calcite Bryozoa M C Calcite Brachiopoda c Exterior, silica, calcite Gastropoda c Calcite Trilobita c Exterior, calcite (Burlington, Burlington, la. Shale Crinoidea c Calcite Upper) Devonian, Livingston Slate Brachiopoda 0 c O slightly carbonized Upper Co., N. Y. C exterior, pyrite (Genesee) Pteropoda c Slate (Chemung) Cohocton, Sandstone Spongiae M C M exterior, argillaceous sand; N. Y. C sandstone Shale Anthozoa O Slightly changed Loan Valley, Crinoidea C Calcite N. Y. Brachiopoda O Slightly changed Devonian, Widder, Can. Shale Anthozoa O Spaces filled with calcite Middle Brachiopoda O Spiralia pyrite; in some cases (Hamilton) absent; shell filled_with cal¬ cite and at times with mud Geneseo, N. Y. Shale Anthoza c Calcite; spaces filled with same * 0 —original; M—mold; C—cast. CONDITIONS OF FOSSILIZATION 285 TABLE I .—Continued Horizon Locality Lithological Character Class Condi¬ tion of Preser¬ vation Remarks Devonian, Crinoidea C Exterior, calcareous , Middle Bryozoa C Calcite (Hamilton) Brachiopoda OM C M and C calcite —Continued Lamellibran- O C C exterior chiata Gastropoda C Exterior and interior in mud; of original in mud Cephalopoda C Calcite; spaces filled with same Trilobita MC Exterior Pratts Falls, Shale Brachiopoda OMC M in shale; C calcite N. Y. Gastropoda C Calcite Cephalopoda C Calcite L. Bethany, Shale Anthozoa C Calcite N. Y. Brachiopoda O Slightly changed Gastropoda OM O slightly changed; M of ex- terior Michigan Limestone Hydrozoa C Replacement; spaces filled with calcite Thunder Bay, Limestone Anthozoa M C Both siliceous; C calcite Mich. Hydrozoa M C M siliceous; C calcite Crinoidea C Siliceous Bryozoa M C M siliceous; C calcite Brachiopoda OMC O slightly changed; C sili- ceous and calcite; M exte- rior and interior Cephalopoda C Siliceous Devonian, Columbus, Limestone Anthozoa C Calcite and spaces filled wi th Lower Ohio same; silica, and spaces (Cornifer- filled with same; also ous) partly calcite and silica; calcite in matrix, but weather out silica Brachiopoda OMC M and C calcite; M exterior, silica, and also calcite Gastropoda M Interior, calcite • Cephalopoda C Calcite Trilobita M Exterior, silica Jeffersonville, Limestone Anthozoa C Calcite Ind. Crinoidea C Calcite Bryozoa C Calcareous Brachiopoda C Calcite, also calcareous Charleston, Limestone Anthozoa C Calcite; Silica Ind. Crinoidea C Siliceous Brachiopoda C Siliceous Silurian, Albany Co., Limestone Anthozoa C Silica Upper N. Y. (Shaley) Bryozoa C Silica (Lower Brachiopoda C Silica Helderberg) Ostrocoda c Silica Trilobita c Silica Limestone Trilobita OMC O slightly changed; M inte- rior calcareous, exterior limestone; exterior and in- terior limestone; also clay; C exterior silica; interior • silica Waterlime Anthozoa C Silica Limestone Brachiopoda OMC O slightly changed; M exter- ior silica; C carbonized Pteropoda C Exterior, limestone Merostomata M C M exterior; shields and seg- ments of abdomen; C car- bonized Albany, Siliceous Anthozoa C Siliceous N. Y. Limestone Bryozoa C Siliceous Brachiopoda C Siliceous 286 J. CULVER HARTZELL TABLE I .—Continued Horizon Locality Lithological Character Class Condi¬ tion of _ Preser¬ vation Remarks Silurian, Jerusalem Water lime Crinoidea O C O slightly changed - , C stems, Upper Hill, Her- Limestone calcite (Lower kimer Co., Helderberg) N. Y. —Continued Cedarville, Limestone Anthozoa C Calcite; silica Herkimer Brachiopoda 0 C C calcareous; siliceous Co., N. Y. Gastropoda C Exterior, calcareous Trilobita 0 Tests of pygidium slightly changed Indian Ladder, Limestone Spongiae C Siliceous Albany Co., Calcareous N. Y. Anthozoa C Siliceous Crinoidea C Ring formed of silica and filled with calcite Bryozoa C Siliceous Brachiopoda C Siliceous (Niagara) Lock port, Anthozoa C Silica; calcite N. Y. Crinoidea C Calcite Brachiopoda O Slightly changed Crustacea O Tests; slightly changed Charleston, Siliceous Anthozoa C Calcite; silica Ind. Limestone Crinoidea C Siliceous Brachiopoda C Siliceous Silurian, New York Shale Graptolitoidea C Carbonized Lower Cincinnati, 0. Limestone Anthozoa C Calcite (Hudson Brachiopoda O Slightly changed River) Cephalopoda c Calcite (Lower Clarksville, Shale Spongiae M C M arenaceous; pyrite; man- Hudson) N. Y. ganese; C spicules re- placed by silica and canals filled with calcite Franklin Shale Spongiae C Spicules and walls of canals Co., Ky. of silica; also calcite; ca- nals filled with chert (Trenton) Kentucky Limestone Spongiae C Silica Foraminifera, the Coelenterates, except the Silicispongia, the Echino- dermata, some of the Vermes, the Molluscoidea, and the Mollusca. Chitin undergoes more or less alteration. In some cases it is replaced by calcite. Silica secreted by organisms is dissolved with comparative ease. It is at times replaced by calcite. The siliceous sponges are very commonly replaced by calcite. If a siliceous organism be found as a siliceous fossil, the original silica has probably been either altered or replaced by silica. Carbonate oj lime is easily dissolved. It is made use of in two forms by organisms. In the form of calcite it is more durable than in the form of aragonite. This is due to the differences in com- pactibility, hardness, and specific gravity. Gastropods, many lamel- libranchs, corals, and other organisms composed of aragonite crumble down and pass into calcite, or disappear, while many composed CONDITIONS OF FOSSILIZATION 287 of calcite may remain. In some strata the aragonite skeletons have entirely disappeared. This is most likely to occur in pervious beds. The presence of calcite forms does not necessarily imply that they were not associated with aragonite forms. The conditions of preservation also vary. In the Mesozoic clays we find cephalo- pods as originals, while in the Palaeozoic clays they are calcite casts. Mytilus edulis secretes aragonite as its inner layer and calcite as its outer layer. Fossils occur in which the inner layer is gone. Cah cite replaces aragonite at times; but in such cases the internal organic structure is gone. As yet no example of aragonite replacing calcite has been reported. Under Table I is given the horizon, locality, and lithological character of the formations studied, and also the class, conditions of preservation, and remarks in connection with certain forms found in these formations. GENERAL MINERAL CHARACTER OF LIVING INVERTEBRATES Foraminijera. —The vitreous and porcellanous forms are calcite. The arenaceous forms are siliceous throughout, or have a sandy- siliceous layer incrusting an interior calcareous layer. The Gro- midae are chitinous. Radiolaria. —Some are composed of acanthine and some of silica. Spongiae. —The Myxospongiae are composed entirely of soft tissues. The Ceratospongiae are made up of spongin fibers. The Silicispongiae are made up of siliceous elements or contain siliceous spicules. The Calcispongiae contain calcareous spicules. Anthozoa. —The Madreporaria are aragonite, and the Alcyo- naria are calcite. Hydrozoa. —The Hydrocorallinae are calcite (?) and the Tubu- lariae calcite (?) and chitin. The Graptolitoidea are chitin. Echinodermata. —Calcite. Vermes. —Calcite ( ?). Bryozoa. —Calcite and aragonite (?). Brachiopoda. —Calcite. Lamellibranchiata. —Some are calcite, some aragonite, and some both calcite and aragonite in layers. Scaphopoda. —Aragonite (?). 288 J. CULVER HARTZELL Gastropoda. —Aragonite. Some are composed of aragonite and calcite. Cephalopoda .—Mainly aragonite. Nautilus pompilius has cal¬ cite for its inner layer and septum, instead of aragonite as hereto¬ fore reported. Crustacea .—Mainly calcite. REPLACING MINERALS The hard parts of invertebrate organisms are composed of more or less soluble mineral matter, and are often replaced by other min¬ erals which fill the cavities left by the hard parts. There may be molecular replacement as the original gradually disappears, or the cavity may be filled by precipitation after the original has entirely disappeared. Chemical reaction may take place, producing new minerals as the elements in the original unite with the elements in the matrix, or elements brought in due to the porosity of the imbed¬ ding material. The imbedding material always contains minerals that are easily dissolved under such conditions as heat, pressure, and moisture, and they may be deposited separately or in combination. The predominating mineral is apt to be found forming molds or casts of the lost parts. In calcareous shales we find calcite casts. In siliceous lime¬ stones we find siliceous casts. In ferruginous formations we find siderite, pyrite, limonite, etc., casts and molds. In galena-bearing formations we find casts composed of that sulphide. These illus¬ trations might be extended; but they suffice to show how the char¬ acter of a formation affects an original skeletal part in its preservation. The most common replacing minerals are calcite, pyrite, silica, limonite, sphalerite, vivianite, barite, malachite, siderite, and hem¬ atite. The list of replacing minerals is quite large, thirty-five being the number. Others undoubtedly occur, and sooner or later will be added to our present list. Under Table II is given the replacing minerals found, and their symbols, Dana’s system being followed in their classification. In the paper to follow will be given a table showing the mineral composition of the more closely related living and fossil forms studied. CONDITIONS OF FOSSILIZATION 289 TABLE II Minerals Replacing Minerals Secreted by Invertebrates 1 Carbonates, Anhydrous: Calcite (CaC 0 3 ), Cerussite (PbC 0 3 ), Mag¬ nesite (MgCQ 3 ), Siderite (FeCO,), Smithsonite (ZnC 0 3 ). Carbonates, Basic Hydrous: Malachite (CuC 0 3 .Cu(OH) 2 ). Chlorides, Anhydrous: Cerargyrite (AgCl). Fluorides, Anhydrous: Fluorite (CaF 2 ). Metals: Copper (Cu), Silver (Ag). Non-metals: Sulphur (S). Oxides, Anhydrous: Cassiterite (Sn 0 2 ). Oxides, Hydrous: Limonite (2Fe 2 0 3 .3H 2 0), Psilomelane (H 4 MnO s (?). Oxides, Sesqui: Hematite (Fe 2 0 3 ). Phosphates, Anhydrous: Apatite ((CaF)Ca 4 P 3 0 I2 ). Phosphates, Hydrous; Vivianite (Fe 3 P 2 Os). Sulphates, Anhydrous: Barite (BaS 0 4 ), Celestite (SrS 0 4 ), Anglesite (PbS 0 4 ). Sulphates, Hydrous: Gypsum (CaS 0 4 . 2H 2 0). Sulphides, Di: Pyrite (FeS 2 ), Marcasite (FeS 2 ). Sulphides, Mono: Sphalerite (ZnS), Galena (PbS), Chalcocite (Cu 2 S), Cin¬ nabar (HgS). Silicates, Hydrous: Kaolinite (H 4 Al 2 Si 2 0 4 ), Giimbelite (Si 0 2 .Al 2 0 3 .Fe 2 - 0 3 .Mg 0 .K 2 0 .Na 2 0 .H 2 0 ), Glauconite (Hydrous silicate of Fe and K), Margarite (H 2 CaCl 4 Si 2 0 I2 ). Silicates, Sub: Calamine (H 2 Zn 2 SiO s ). Silicon, Oxides of: Flint (Si 0 2 ), Silica (Sio 2 ), Sand (Si 0 2 ). REFERENCES TABLE II A Bernhard, Elements de Paleontologie, 1895. Geikie, Text-Book of Geology, 3d ed., 1893. Gratacap, American Naturalist , Vol. XXXI, No 363. Nicholson and Lydekker, Manual of Paleontology, 3d ed., Vol. I, 1889. Reis, “Ueber Petrifikation der Muskel,” Archiv fur mikroskopische Anatomic, Vol. XLI. Trabucco, La Petrificazione, 1887. Von Zittel, Handbuch der Paldontologie. White, Bulletin of the United States Geological Survey, Vol. I, No. I, Art. 8, 1879. - Bulletin of the United States Geological and Geographical Survey of the Territories, Vol. V, 1880. 1 The writer is indebted to the late Professor Beecher for valuable material from his private collection; to Rt. Rev. Joseph C. Hartzell, Bishop of Africa, for material from that continent; to Rev. Morton Culver Hartzell, for material from Germany; and to Major Ebenezer Thresher, for material from France. • ; .- J f , ' . 1' > A) ... - . V . v V '' v > ■ '■ « !'■ T.i ;'<«♦!.. : ■ - •v. l . : , • -■'^S ■' ,•* .--V ■ y*Y . 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