Not much attention has been given to the fossil invertebrates and plants. It is evident that neither the mollusks nor the plants have undergone any considerable change during Pleistocene times and are therefore not available as indicators of geological stages, though often useful for determining local climatic conditions. Their value can be better utilized by the palæomalacologists and palæobotanists.
To the officers of museums and colleges and to the private individuals who have so freely offered the use of their materials and in other ways aided the writer, he takes pleasure in expressing his sincere thanks. Most of all, however, he is indebted to the Carnegie Institution of Washington for the generous support extended during the years of this investigation.
THE PLEISTOCENE OF NORTH AMERICA AND ITS VERTEBRATED ANIMALS.
CONCLUSIONS REGARDING THE DIVISIONS OF THE PLEISTOCENE.
The Pleistocene is regarded as being equivalent to what is known as the Glacial period. It began with the deployment of the ice-sheets which, proceeding from their centers of accumulation in British America, laid down in the East the Jerseyan drift and in the West the Nebraskan. The more the Glacial period is studied the more one becomes impressed with the significance of its physical effects on the northern hemisphere and with its influence on the vertebrate life. Doubtless its effects on the world in general are only beginning to be comprehended. The writer knows of no other phenomena, geological or biological, which so well characterize the Pleistocene period as do those comprehended under the term Glacial. They constitute the key to the determination of the subdivisions of the epoch and of their succession and to the history of the vertebrates which during this time occupied the continent.
The Blanco is held to belong to the upper, or to the uppermost, Pliocene. It is at present assigned to the Middle Pliocene (Osborn, Bull. U. S. Geol. Surv. No. 361, p. 81; Matthew, ibid., p. 120). Until recently the oldest known Pleistocene vertebrates appeared to be represented by the collections which long ago were made at Fossil Lake, Oregon, and at Grayson (Hay Springs), Nebraska. These assemblages had formerly been referred to the Pliocene, and the belief that they belong there is not yet wholly without supporters. It seemed, therefore, proper to retire the Blanco somewhat. The discovery that the Fossil Lake and Grayson faunas were represented in the Aftonian deposits of Iowa, and belonged probably to the first interglacial stage, reveals the fact that the geological interval between the Blanco and the Aftonian is at least partly filled by the first glacial stage, the Nebraskan. Naturally, it is to be expected that the breach between the earlier and the later faunas will be occupied, in part at least, by the vertebrates of the Nebraskan. What these are is not yet well determined; but the writer believes that as the Blanco and its equivalent and closely related formations and faunas become better known, they will be attracted close to the Pleistocene.