PALEONTOLOGICAL GEOLOGY treats of the structure, affinities, classification, and dis tribution in time of the forms of vegetable and animal life imbedded in the rocks of the earth's crust, dealing chiefly with fossils. The first question that naturally suggests itself is: How came the remains of plants and animals to be preserved in these old rocks? If we argue from the conditions of the present day, and suppose such condi tions to have prevailed in the ancient time, we are not satisfied or enlightened as to such remarkable preservation. The conditions for the preservation of any relics of plant or ani mal life of a terrestrial surface must be always exceptional. They are supplied only where the organic remains can be protected from the air and from superficial decay. Hence fos sils may be found in lakes over the bottoms of which deposits of silt, peat, marl, etc., are formed. Peat mosses are more favorable still for the preservation of ancient fauna. In them wild animals have sunk, and the antiseptic quality of the peat has preserved them from decay. Fauna and flora may be buried in the sand and silt of deltas at the mouths of rivers. Caverns are particularly adapted to the preservation of the higher forms of animal life. Most of our knowledge of the pre-historic mammalian fauna of Europe is derived from What has been disinterred in bone caves. These caves serve as dens for predatory beasts. into which they dragged their prey. In some cases they were merely holes into which animals crawled to die, or in which they were deluged and drowned. The bottom of the sea below tide-mark is favorable to the preservation of marine organisms; and the fossils of the ocean are vastly greater in variety and num ber and of much more importance in geological study than those of the land. As to the relative age of foseils, although absolute dates cannot be fixed in geological chronology, it is not difficult to determine the relative age of different strata, and consequently of their inclosed organic remains. For this purpose the fundamental law is based on what is termed the "order of superposition." This law may be thus defined; In a series of stratified formations the older must underlie the younger. It is not needful that we should actually see the one lying below the other. If a continuous conformable• sue. cession of strata dip steadily in one direction we know that the beds at the one end must underlie those at the other, because we can trace the whole succession of beds between them. The true order of superposition is decisive of the relative ages of stratified rocks. There is nothing in fossils themselves to fix their date; but it is certain that those in the younger strata were ages later in appearance on the earth than those in the older strata. There are two main purposes to which fossils may be put in geological research: 1, to throw light upon former conditions of physical geog raphy, such as the presence of land, rivers, lakes, and seas, in places where they do not now exist; changes of climate, and the former distribution of plants and animals; and, 2, to furnish a guide in geological chronology whereby rocks may be classified accord ing to relative date, and the facts of geological history may be arranged and interpreted as a connected record of the earth's progress. Former land surfaces are revealed by the presence of stumps of trees with .roots branching freely in the underlying stratum, which, representing the ancient soil, often contains leaves, fruits, and other sylvan remains, together with traces of the bones of land animals, remains of insects, land shells, etc. Ancient woodland surfaces. of such character are found between tidemarks, and even below low-water line. They unequivocally prove a subsidence of the land. The former existence of lakes can be satisfactorily proved from beds of marl or lacus trine limestone, full of fresh-water shells, or from .fine silt with leaves, fruits, and insect remains. Such deposits are abundantly forming at the present day, and they occur at various horizons among the geological formations .of past times. Old sea-bottoms are vividly brought before us by beds of marine shells and .other organisms. Layers of water-worn gravel and sand, with rolled shells of littoral and infra-littoral species, unmistakably mark the position of a former shore line. Deeper water is indicated by finer muddy sediment, with relics of the fauna which prevailed beneath the reach of waves and ground-swell. Limestones full of corals, or made up of crinoids, point to the slow continuous growth and decay of generation after generation of organisms in clear sea water. The existence of different conditions of climate iu former geologi cal periods is satisfactorily demonstrated from the testimony of fossils. Thus an assem blage of the remains of palms, gourds, and melons, with bones of crocodiles, turtles, and sea-snakes, proves a sub-tropical climate to have prevailed over the s. of England in the time of the older tertiary formations. On the other baud, the presence of au intensely cold or arctic climate far s. in Europe during•post-tertiary time, can be shown from different kinds of evidence, such as the existence of the remains of arctic animals even as far as.France. Observations made over a large part of the globe have enabled geoligists to divide the stratified part of the earth's ,crust into systems, formations, and groups, or series; and it is by their characteristic fossils that the divisions of stratified rocks can be most satisfactorily made, since each formation, being followed and distil*. guished by its own assemblage of organic remains can be followed and recognized even amid the crumblings and dislocations of a disturbed region. It was at one time believed, and many still hold, that groups of strata characterized by community or resemblance of organic remains were chronologically contemporaneous. But such an inference rests• upon most insecure grounds. We may not be able to disprove the assertion that the strata were strictly coeval, but we have only to reflect on the present conditions of zoological and botanical distribution, and of modern sedimentation, to be assured that the assertion of contemporaneity is a mere assumption. Consider what would happen were the present surface of any portion of southern Europe to be submerged by the sea, covered by marine deposits, and then re-elevated into land. The river-terraces and la.custrine marls formed before the time of Julius Ctesar could not be distinguished by any fossil tests from those laid down in modern days, unless traces of human imple ments were obtainable,whereby the progress of civilization during 2,000 years might he indicated. So far as regards the shells, bones, and plants preserved in the various forma
lions, it would be absolutely impossible to discriminate their relative dates; they would be classed as "geologically contemporaneous," that is,,as. having been formed during the same period in the history of life in the European area; yet there might be a differ ence of 2,000 years or more between many of them. Strict contemporaneity cannot be asserted of any strata merely on the ground of similarity or identity in fossils. Still it may be true that lake or sea deposits of widely different characteristics may have been contemporaneous in the time of their deposition, as a lake bottom in America might show certain types of fossils, while an entirely different assortment might at the same period have been settling into the bottom of a lake in Asia. But the grand march of life, in its progress from lower to higher forms, has unquestionably been broadly alike in all quarters of the globe. But nothing seems more certain than that the rate of advance has not everywhere been the same. It has moved unequally over the same region. A certain stage of progress may have been reached in one quarter of the globe thousands of years before It was reached in another; though the same general succession of organic, forms inigbt be found in each region. The geological formations form the records of these ages of organic development. In every country where they are fully displayed and have been properly examined, they can be separated from each other according to their organic contents. Their relative age within a limited geographical area can be demonstrated by the mere law of superposition. 'Where, however, the formations of distant countries are composed, all that we can safely affirm regarding them is that those containing the same or a representative assemblage of organic remains belong to the same epoch in the history of biological progress in each area; but we cannot assert that they are contemporaneous, unless we are prepared to include within that term a vague period of perhaps thousands of years. Gaps ins the geological record show that the history of life has been very imperfectly pre served in the stratified parts of the earth's crust. Apart from the fact that, even under the most favorable conditions, only a small proportion of the total flora and fauna of any period could be preserved in the fossil, enormous breaks occur where no record has been preserved at all. It is as if whole chapters and books were missing from a histor ical work. Fossil evidence, moreover, may be made to prove the existence of gaps which arc not otherwise apparent. Changes in organic forms must, on the whole, have been extremely slow in the geological past. The whole species of a sea-floor could not pass entirely away and be replaced by other forms without the lapse of long periods of time. If then among the conformable stratified formations of former ages we encounter sudden and abrupt changes in the fades of the fossils, we may be certain that these must mark omissions in the record, which we may hope to fill in from a more perfect series elsewhere. There have never been any universal interruptions in the continuity of the chain of being, so far as geological evidence can show. But the physical changes which caused the breaks may have been general over a zoological district or minor region. They no doubt often caused the complete extinction of genera and species which had a small geographical range. It is therefore clear that time geological record, as it now exists, is at the best but an imperfect chronicle of geological history. In no country is it complete. The lacunm of one region must be supplied from another; yet in proportion to the geographical distance between the localities where the gaps occur and those whence the missing intervals are supplied, the element of uncertainty in our reading of the record is increased. The most desirable method of research is to exhaust the evidence for each area or province, and to compare the general order of its succes sion as a whole with that which can be established for other provinces. It is therefore only after long and patient observation and comparison that the geological history of different quarters of the globe can be correlated. Subditisions of time geological record are made by fossils, and they are made the bases for the geological classification of rocks. Thus we may find a particular stratum marked by the occurrence in it of various fos sils, one or more of which may be distinctive, either from occurring in no other bed above and below, or from special abundance in that stratum. These species might be used as a guide to the occurrence of the bed in question, which might be called by the name of the most abundant species. In this way a geological horizon or zone would be marked off, and geologists would thereafter recognize its exact position in the series of foimations. The first and fundamental point is to determine accurately the order of superposition of strata. Until this is done, detailed paleontological classification may prove to be worthless. But when once the succession of the rocks has been fixed, paleontological evidence may become paramount. It cannot be too frequently stated nor too prominently kept in view that, although gaps occur in the succession of organic remains as recorded in the rocks, there have been no such blank intervals in the progress of phi& and animal life upon the globe. The march of life, onward and upward, has been unbroken. Geological history, therefore, if its records in the stratified formations were perfect, ought to show a blending and gradation of epoch with epoch, so that no 'sharp divisions of the events could be made. But the progress has been constantly interrupted, now by upheaval, now by volcanic outbreaks, now by depression. These interruptions serve as natural divisions in the chronicle, and enable the geologist to arrange his history into periods. A bed, or limited number of beds, characterized by one or more distinct fossils, is termed a zone or horizon, and is often known by the name of a typical fossil, as the different zones in the Lias are by their special species of ammonite. A series of such zones, united by the occurrence among them of a number of the same species or genera, is called a group. A series of groups similarly related constitute a formation, and a number of formations may be united into a system.