Page:Popular Science Monthly Volume 76.djvu/595

 than are recognized for invertebrates, because of the scattered nature of the material and the additional probability that continental deposits, in which alone vertebrates have their chief importance as guide fossils, have accumulated more rapidly than marine beds. Similarly, conditions peculiar to their mode of deposition make it difficult, perhaps impossible, to define lithologically the limits of the zones we are attempting to characterize. And here another trouble confronts us, for the faunas are incompletely known, and we are not yet in a position to dogmatize too freely on the subject of vertebrate index fossils. But that the method of zonal studies is the correct one is very clearly shown in Dr. Matthew's recent monograph on the Carnivora and Insectivora of the Bridger Eocene, and will be demonstrated with equal force when Professor Osborn's volume on the titanotheres is published.

Various attempts have been made at the correlation of European and American mammal horizons, their measure of success depending entirely on the degree of closeness with which these correspond to true zones. At present, we are attempting to correlate subdivisions, both faunal and stratigraphic, of all orders of magnitude, the majority including many faunules and many zones. Evidently, this tendency must be corrected by careful zonal studies, if vertebrate paleontology is to have any standing as an aid to stratigraphy in the correlation of our non-marine formations.



N deciphering the ancient geography as to the position of the marine waters and the land masses, we as pioneers in this work must be controlled primarily by the known fossilized life and secondarily by the character and place of deposition of the geologic formations. This record is most extensive and best preserved in the deposits of the continental and the littoral region along the continental shelves of the oceanic areas. Back of these two principles, however, there is another that eventually will become the primary guiding factor. It is the principle of diastrophism—one seeking to explain the causes for the periodic movements of the lithosphere.

In our study of the ancient seas with their sediments and entombed life we have safe guidance in the phenomena of the present. Ludwig in 1886 estimated the species of animals then known to naturalists as upwards of 312,000, and in 1905 Stiles thought this great total had increased to about 470,000 forms. Of this sum fully 60 per cent, are insects, and of the remainder, the writer concludes that about 25 per