and recent species are such as to leave no doubt on the mind that the same harmony of parts and beauty of contrivance which we admire in the living creation, has equally characterized the organic world at remote periods. Thus as we increase our knowledge of the inexhaustible variety displayed in living nature, and admire the infinite wisdom and power which it displays, our admiration is multiplied by the reflection that it is only the last of a great series of pre-existing creations, of which we cannot estimate the number or limit in times past."1

To this I have only one thing to add. All that we now-thanks to the discoveries of astronomers-know of the wonders of the starry heavens, is much more fitted to give us an idea of His grandeur whose glories the heavens declare, than was the scanty and limited knowledge of our forefathers; and in the same way our knowledge of the animal and vegetable world which clothed and peopled our earth in the primeval age, if it is extended and made definite by the progress of paleontology, will make the might, wisdom, and goodness of the Creator more overpoweringly evident to us than can be done by the existing creation.

1 Elements of Geology, p. 772.

XV.

THE PALEONTOLOGICAL HISTORY OF THE EARTH

BEFORE I can discuss the relation of those results of geological inquiry which deal principally with palæontology, to the Mosaic account of creation, I must enter more fully into certain points which were only touched upon in my last lecture; and this in order to facilitate the review of the whole subject. The first of these is the use of fossils in determining the limits of the separate formations of the earth's crust, and their relation to each other.

I have already discussed the general division of rocks into stratified and unstratified, and I have told you that no fossils are found in the latter. The great mass of unstratified rocks which underlie the stratified rocks are supposed to be the oldest portions of the earth's surface, and these rocks are therefore called primitive. It is commonly supposed by geologists that the stratified rocks which are found in parallel layers, superposed above them, were gradually deposited by water. Werner called this part of the earth's surface the sedimentary rocks, in contradistinction to the primitive rocks which are always found below them, and to the alluvial soil which is always found above them. The lowest rocks, i.e. those sedimentary rocks which lie next to the primitive rocks, he called transition, and

he divided the other sedimentary rocks into the older, middle, and recent formations. In other countries the rocks lying on the primitive rocks were divided into three groups; the transition rocks, with one or two of the formations lying nearest to them, were called primary, the greater portion of the sedimentary rocks secondary, and the strata nearest the surface tertiary rocks. For the present I may omit other names and divisions. But each of these principal divisions contains several different sub-divisions or strata, which are called by different names, partly from their constituent parts, as coal formation, chalk formation, etc., Lias, the English name for a kind of limestone, the Triassic formation, because it always consists of three component parts, keuper, muschelkalk, and red sandstone or bunter sandstein; partly from the places in which they are found, as, for instance, the Silurian formation, which is named from the district in the West of England which the Silures are supposed to have inhabited in the time of the Romans, the Devonian formation, so called from Devonshire, Permian formation, from the Russian province Perm, Jurassic formation, from the Swiss and French mountain Jura, and so on.

One of the principal tasks of geology was, first to define the limits of the separate formations, and then to determine their relative age; and also, especially, to discover which different strata are parallel to each other in different regions, that is, probably belong to the same period. In order to accomplish this, it was first necessary to ascertain the materials of which they were composed, and the order in which they were

deposited; afterwards the fossils occurring in them were examined, and of late this last point has been specially and almost exclusively considered.1

In the course of these researches places were observed in which several strata were found resting on each other, without any evidence that a disturbance of the original order of deposit had taken place. In such cases the lower rocks must of course be considered as the older, and those nearer the surface as the more recent rocks. When the fossils contained in the separate strata were examined, it was found that certain fossils were peculiar to certain strata, that they were found only in one particular stratum, not in one above or below it, not in an older or a newer one. This was observed in several places, where a clear and undisturbed succession of strata existed. A series of such observations proved that certain fossils were characteristic of certain strata, and that the relative ages the strata in question could be ascertained from the presence of these fossils. In one place, e.g., three strata were found superposed, and each contained certain characteristic fossils; we will call them A B C, beginning from the lowest. In another place the same group was found in the same order; in a third there was one stratum above them, D; in a fourth the lowest stratum was missing, but B C D were found in the same order; in a fifth A CD, etc. Supposing that all the strata

1

of

Zittel, Aus der Urzeit, p. 53. The first person who made use of fossils for this purpose (about 1800) was William Smith, "the father of English geology," also called Stratum-Smith. See H. Miller, Testimony of the Rocks, p. 119. J. P. Smith, The Relation, p. 55. Cotta, Geologie der Gegenwart, p. 37. Peschel, Gesch. der Erdkunde, p. 621. Zittel in the Historischer Taschenb. p. 162.

which exist, and which are characterized by possessing peculiar fossils, were named after the different letters of the alphabet, we should, no doubt, never find the whole series from A to Z together anywhere; generally there are only a few letters, as, for instance, in one place we find A B E F, with C and D wanting; in another B D E, with A and C wanting, but never A B D C, so that D is found under instead of above C. Those fossils which are common to certain strata in different regions, and are also peculiar to those strata, never occurring in earlier or in later ones, and which are therefore characteristic of the formation in which they are found, are called guiding fossils or shells, because they guide geologists in classing the formations in which they are found.

After this law had been confirmed by numerous observations in places where the succession of the strata is undisturbed, geologists were justified in applying it to those places in which the geognostic conditions were not so simple and clear; and thus in making use of the fossils, as I pointed out in my last lecture, as medals or inscriptions in the strata of the earth's crust. We are enabled thus by the help of the fossils to ascertain with tolerable certainty the simultaneous origin or comparative age of deposits which occur at considerable distances from each other, and which are of different mineral composition. Only, in making these calculations, it must not be forgotten that probably fresh water as well as sea water was always in existence, so that the deposits which contain the remains of animals living on the land, or in fresh water, must be distinguished from contemporary sea deposits;

and