the forests grew and matured; water would have to cover the same area to deposit the succeeding stratum; and again this stratum would have to rise above the water before a second forest could grow. There are two ways in which this result could be brought about. In a district subject to such intense volcanic action as this must have been, a succession of minor oscillations might have been associated with the general subsidence, so that large areas of the lake border districts would be alternately above and beneath the sea, or, as was doubtless often the case, the shallow portions of the sea became filled up with the rapidly accumulating ejecta, and sub-aërial deposits of sufficient depth were laid down to allow the growth of forests, which, in time, were depressed by the general subsidence, to be buried by a succeeding stratum of the volcanic débris. But this latter method was not the ordinary one, as is attested by the fact that many of the forests have grown in beds of finegrained material that must have been formed beneath the surface of the water.

I shall, however, not attempt to pursue this matter farther until all the data and materials collected have been examined. A thorough study of the various volcanic rocks will probably throw much light upon this very interesting group of strata.

Art. VIII.-Paleontological Papers No. 10: Conditions of Preservation of Invertebrate Fossils.

By C. A. White, M. D.

During the prosecution of his field-work, the paleontologist often observes certain interesting relations, not only between the lithological character and composition of the different kinds of fossiliferous rocks and the condition in which their contained fossils are preserved, but also between the lithological composition of those strata and the faunal characteristics of their fossils. In the latter instances, the case is largely one of original character and condition of the sea-bottom sediments as the ground of the habitat of those animals, while they were living, whose fossil remains they as rocky strata now inclose. It is proposed to make this latter subject the basis of a future paper, but the present one will be devoted to a discussion of some of the conditions of fossilization and preservation of invertebrate remains as they are found in the various kinds of stratified rocks of the different geological ages, and to institute some comparison of their mineral composition as fossils with that which they possessed in the living state, assuming that of the latter by the known composition of their present living representatives. It would add greatly to the interest and scope of this subject if a series of careful chemical analyses of these substances, both fossil and recent, could form a part of the basis of its discussion, but no opportunity has yet occurred for accomplishing such a task. Although detailed chemical analyses do not enter into the data for these discussions, a consideration of the mineral composition of the fossil remains as they now exist, and a comparison of that composition with what it was in the living state, necessarily forms the basis of a considerable part of the present paper. Such comparisons, while they show a close similarity in a very large proportion of cases, always exhibit at least some degree of contrast; and in some cases, a total change of mineral composition is found to have taken place. Many of the facts herein stated are patent to every collector of fossils, but the subject to which they relate is seldom discussed in paleontological writings, and the few references that are made to it are usually of a special or local character. Therefore in view of the somewhat extended field observations of the writer in the median portion of North America, it has been thought advisable to embody some general observations upon this subject in tlre present paper.

In a general way we may divide the substances which in life constituted the skeletal parts of invertebrate animals, which parts alone have

been preserved in a fossil state, into the chitinous, siliceous, and calcareous. Chitinous skeletal substances are almost wholly, if not entirely, confined to the Articulata (including the lyopomatous brachiopods) and certain Acalephs; the siliceous, excepting the teeth of certain mollusks, to the Protozoa; while, with the exception of all but a few of the Articulata, all other invertebrates possess skeletal parts, if any, which are composed of calcareous substance only as their mineral constituent. These remarks, of course, apply to the composition of those skeletal parts in their living state. The mineral change which some of them have undergone after their first fossilization will be considered in following paragraphs. In fact, then, we find that the original mineral composition of far the greater part of invertebrate fossils was, in the living state, lime-carbonate combined with a greater or less proportion of animal substance; having been the same indeed as that of the shells. and corals which constitute their present living representatives. Since the animal substance of these bodies, as is also the case with bones, was displaced during the process of fossilization, usually by the infiltration and precipitation of additional lime-carbonate, it would seem that the chances for, and the condition of, the preservation of all kinds of these calcareous skeletal remains would be equal in identical circumstances of environment. But there is, as we shall see, a greater variety of condition among fossil forms of this kind than is referable to either their original mineral composition or the environing circumstances of their fossilization and preservation.

Three divisions may be made of the manner in which fossil forms are preserved, namely: (1) fossilization proper of the substance of the skeletal parts; (2) moulds; (3) casts; (4) pseudomorphs. In the first case, the original substance, as well as the external form of the object, is more or less completely preserved. In the second, the whole substance of the object, at some time subsequent to its rocky entombment, has been removed by decomposition or solution, and its consequent escape through the porous imbedding matrix, leaving a cavity which is the exact counterpart of the external form of the formerly inclosed object. In the third case, the more or less complete external form of the object has been restored by the refilling of the mould with a substance similar to that of the matrix, or the deposition of some mineral either wholly or in part different from the matrix; such as calcite, pyrite, &c. In the fourth case, that of pseudomorphs, the substance of the fossil in its original condition has been exchanged, atom by atom, for another mineral substance, usually silex.

Pseudomorphs thus resemble casts, and in one sense they are such, but they differ from casts, which consist merely of the filling-substance of previously formed moulds, by the manner of their production, and in many cases, at least, by having the texture and even the microscopic structure of the original body preserved. This method of preservation is perhaps more clearly exemplified in the case of silicified

wood than in any other, where not only the character and appearance of the woody fibre is perfectly retained, but the microscopic structure is often so perfectly preserved as to distinguish the botanical divi sion to which it belongs. In the case of pseudomorphic silicification of invertebrate fossils, the microscopic structure is seldom, if ever, so perfectly preserved as it is in the case of wood, but their true pseudomorphism, nevertheless, occurs, the more common cases of which consist of the silicification of shells, corals, &c., in limestone. In these cases, the objects were without doubt first fossilized as calcareous substances, and as such they could not have been removed by solution, leaving their moulds in the matrix, because that matrix was of essentially the same composition as themselves and nearly or quite equally soluble. Therefore it is evident that the change of substance must have been effected by the process of pseudomorphism which has just been explained, and which was initiated by a movement and segregation of the siliceous atoms that were previously distributed in the substance of the imbedding rock, which movement and segregation were much the same as took place in the formation of flint-nodules in chalk and similar siliceous nodules and masses that are often met with in limestones. That this pseudomorphism of calcareous fossils by silicification began long after they had become perfectly fossilized, is indicated by the fact that the process has, in most cases at least, taken place in connection with the weathering of the strata which contain the fossils. For example, it is very common to find shells, corals, crinoid fragments, &c., wholly or partly silicified upon and near the surfaces of certain paleozoic limestone strata, as well as in the interstices which are filled with the débris of their decomposition; while fossils of the same kinds which are imbedded in the solid and unweathered portions of the same strata, are wholly calcareous. Although fossilization proper of calcareous remains is more complete in limestones than in other rocks, it is in limestones also that most of the cases of silicious pseudomorphism of invertebrate remains, as just defined, takes place, the latter cases as compared with the former being very few.

Casts and moulds are more common in sandstones than in other rocks, although they are not uncommon in magnesian limestones, and in both sandy and argillaceous shales. The common occurrence of moulds in sandstone is easily accounted for by the porous character of the rock, which admits of the percolation of water charged with solvents of limecarbonate. Such percolation is, of course, necessary in the production of moulds in all kinds of rock, but it is more complete in sandstones than in any other rocks. Some of the finest moulds of both fresh-water and marine species, those which preserve not only the form, but also the delicate surface-markings of the fossils most perfectly, are found in hematite; but such instances are very rare compared with the occurrence of moulds in sandstones. While ordinary limestones are, as a rule, more fossiliferous than any other rocks, magnesian limestones notably

contain very few fossils. There are some exceptions to the latter rule, but in all cases, so far as I am aware, when the rock approaches a true dolomite, the contained fossils are all and of all kinds in the form of moulds, the inner surfaces of which usually have a drusy lining.

Casts of the more perfect kinds are formed by the infiltration of foreign mineral matter into complete moulds which were previously formed in the manner already explained, and by its precipitation there until the cavities became completely filled. Such casts are sometimes composed of iron pyrite, some of which are often very perfect and beautiful. More frequently, however, the casts are such as have been formed by a filling of the spaces which the fossils formerly occupied by the substance of the rocky matrix in which they were imbedded, the fossils themselves having gradually passed away by solution, while the substance of the adjacent matrix followed closely upon it in consequence of the pressure of the superincumbent strata. It is in the form of such casts as these alone that many fossils, especially the shells of several families of Conchifers, have been preserved; and single valves of such shells are often very well preserved in that manner.

The manner of preservation of invertebrate fossils which is here designated as fossilization proper consists of the minimum of change in the mineral constituents of the skeletal parts. As already stated, some change is always effected in the mineral composition of the object fossilized, but almost all molluscan as well as certain other shells, and also corals, being composed almost wholly of lime-carbonate, when preserved in limestone strata, or in such strata as contain any considerable amount of lime-carbonate, still retain in the fossil state much the same composition which they had while living. In most of these cases, the change has been so slight that even the microscopic texture of the shells and the minute and intricate details of the corals are as perfectly preserved as they were in the living state. Thus, the fibrous shell-structure of certain families of Brachiopods, and the punctate structure of others, the pearly shell-structure of certain Cephalopods, Gasteropods, and Conchifers, and the prismatic shell-structure peculiar to certain families of Conchifers, are often, indeed generally, as easily studied in the fossils as they are in their living representatives.

The siliceous skeletal parts of fossil invertebrates, when such were produced, seem to have been so purely siliceous that they have apparently not changed at all by fossilization. Chitinous substances, however, seem always to have undergone more or less alteration, although they are among the more permanent of fossil forms. For the purposes of this discussion, under the head of chitinous fossils are included the shells of lyopomatous brachiopods (such as those of Lingula, Discina, &c.), the crusts of trilobites and other crustaceans, and stipes of graptolites. In limestones, chitinous fossils seem to have become quite calcareous in some instances, and in all cases they are more brittle and mineral-like than living chitine is, and yet in all cases they are readily recognized as