24 
ANNALS NEW YORK ACADEMY OF SCIENCES 
details did not appear until 1846, when they were published as Part 3 ofthe 
“Geological Observations.” The whole subject was discussed attractively 
in the second edition of the Journal of Researches. 
The superficial deposit of the great plains is a “reddish argillaceous 
earth,” containing concretions of indurated marl, which at times become 
continuous layers or even replace much of the red earth. In the northerly 
part of the plains area, this pampas deposit, which passes downward into 
sands, limestones and clays of late Tertiary age, yielded no marine shells to 
Darwin; its infusoria, studied by Ehrenberg, proved to be partly marine, 
partly freshwater, while the marly concretions resemble some freshwater 
limestones seen in Europe; but this paucity of invertebrate life was unim¬ 
portant, for the whole of that region proved to be one vast cemetery, in which 
the skeletons of gigantic extinct mammals are so numerous that a line could 
not be drawn in any direction without passing through some bones. In 
northern Patagonia the red deposit is bound closely to an overlying gravel, 
containing marine forms belonging to species now existing on the coast, 
while in southern Patagonia marine shells occur in the pampas deposit itself. 
Darwin believed that this pampas material was deposited within a vast 
estuary, into which great rivers carried from the surrounding region carcasses 
of the animals whose skeletons were entombed in muds tranquilly accumu¬ 
lating on the bottom. All conditions go to show that the mammalia became 
extinct after the sea had received its present fauna, and there is nothing to 
suggest that a period of overwhelming violence swept away and destroyed 
the inhabitants of the land; everything supports the contrary belief. The 
only noteworthy change in conditions has been a gradual elevation of the 
continent; but that was not enough to modify the climate or to bring about 
a change in the land fauna. 
Several of the important genera collected by Darwin had been found 
in North America long prior to his time. This similarity of the Quaternary 
faunas induced him to speculate on the causes which had divided the Amer- 
can continent into two well-defined and somewhat contrasting zoological 
provinces. He does not hesitate to suggest recent elevation of the Mexican 
platform or, more probably, recent submergence of the West Indian Archi¬ 
pelago as a conceivable cause of this separation. It seems to him most 
probable that the elephants, the mastodons, the horses and the hollow¬ 
horned ruminants of North America “migrated, on land since submerged 
near Bering Straits, from Siberia into North America, and thence, on land 
since submerged in the West Indies, into South America, where for a time 
they mingled with forms characteristic of that southern continent and have 
since become extinct.” Had this American Museum of Natural History 
existed in Darwin’s day, study of the remarkable exhibits in its Mammal 
