Page:The American Cyclopædia (1879) Volume XII.djvu/823

to admit the carrying on of the same process, the operation of the same causes, possibly at some times more intensified, through indefinitely long periods, to produce in the greater number of instances the results which we see. The process of fossilization gradually supervening, with the induration of the entire enclosing mass of material, we have the beds of sand, clay, and calcareous mud converted into sand- stone, shale, and limestone, still enclosing the same organisms as when they formed a part of the ocean bed; and it may be regarded as certain that these deposits were originally in horizontal or essentially horizontal position. Many of these fossil organisms were living in the condition, association, and arrangement in which we now find them. Others have been transported, broken, and mingled with those of the undisturbed beds. In some instances myriads of individuals of various forms have lived and died upon the sea bed, and have remained long undisturbed and not covered by sediments, since we find them sometimes supporting and enveloped by some other organism, as a coral, a bryozoan, or the roots of crinoidea. The physical nature and condition of the older metamorphic strata, in which we have no remains of animals (either they not then existing, or having been obliterated by physical and chemical changes), prove the conditions of sea and land to have been essentially the same then as at present. The great extent of ancient limestone formations renders it probable that the ocean of that period was inhabited, although of the inhabitants there is no trace. From the period of the Potsdam sandstone in America and the Cambrian rocks of Europe, we find the remains of numerous forms of animal and vegetable life. We see traces of the conditions under which they lived; the sands of the sea beaches, laid bare by the ebbing tide, were rippled by the wind and trailed by the animals of that primeval ocean. From that period at least, light and heat, cloud and sunshine, rain and wind refreshed and fertilized the earth, which teemed with animal and vegetable existences. The testimony of living things is found in their fossil imprints, the earliest evidences of life in the remains of plants and animals imbedded in the ancient sea bottoms or stranded upon the shores. Through unnumbered ages life has presented its varied forms without cessation from its first appearance on the globe; each successive epoch, each new physical condition, whether of ocean bed or shore, of moist or of dry land, presenting its new and peculiar fauna or flora. In the course of these incalculable periods the aspect and character of the existences have changed, and there has been, if not a regular progression, yet in the main a wonderful advance over the earlier organisms. The relations of these fossiliferous beds one to another have led to the recognition of geological periods; and these periods are verified over wide areas of country, continuously or interruptedly, even to the extent of the two hemispheres. It is from the occurrence of certain peculiar forms, or an assemblage of them, in these strata, that the period or geological age can be determined.—A remarkable feature which strikes the observer is the great number of types that have appeared in a limited geological period. Trilobites, for example, lived in epochs of limited range; also the great reptiles, the pterodactyls, and certain forms of fishes, as well as some of the lower organisms. Many of these are so peculiar that their appearance or disappearance is at once noticed. They are wanting in the most ancient epochs; afterward they are developed in abundance, and more or less gradually disappear, leaving no trace in subsequent periods. This is equally true of all the other forms; and scarcely any extend over more than a small number of geological periods. In the most ancient epochs the greater number of forms have not existed; there we would search in vain for fishes, reptiles, birds, and mammifers. All our fossil fishes belong to geological formations relatively recent. In the mollusks, the articulates, and the radiates, the greater number of species appear after the ancient epochs, and successively in each of the subsequent ones; while a few types, beginning in the earlier geological periods, have been continued to the recent, or even to present times. This is notably true of the linguloid type, although not of the true lingula. Other species have had a brief existence. Created early, they disappear before the recent epochs, and in the fact of their early appearance, as a rule, is the certainty of their prompt disappearance. Some forms which lived in the intermediate epochs are completely wanting in the older and in the newer formations. In regard to the number of fossil genera which have had a limited duration, M. d'Orbigny enumerated, from about 1,600 then known, only about 16 which occur in all the periods; and while the whole number of genera has been since that time greatly augmented by new discoveries, a critical study will probably prove that fewer than 16 are common throughout the whole. It sometimes happens that one or more species may appear at an earlier epoch, rare or common, but usually restricted to a limited area, and, having disappeared for an interval of one or two epochs, may reappear in greater numbers and over a far wider extent, and in association with an entirely different fauna from the first; but such cases are exceptions. M. Barrande has designated species in this mode of occurrence as colonies, and has shown that colonies are of common occurrence in the palæozoic rocks of Bohemia. As a rule, the extinction of a fauna at the close of any geological epoch depends upon the degree of change in the sediments. Where the change is abrupt, the fauna is more likely to be entirely cut off; while if the change be in less degree, some of the species may survive. Again, where the fauna is apparently destroyed, from a change in the character of the sedi-