18 The American Naturalist. [January, 
Chaco. Borings and chemical analysis of the soil may here- 
after give us reliable data; but in the meantime we may safely 
reckon that an area of 200,000 square miles has been mainly 
formed from the materials derived from the ancient mount- 
ains.” 
In the west, in Wyoming, Nebraska, and Montana, there ex- 
isted in tertiary times large fresh-water lakes,’ the success ors 
to the wide cretaceous seas which before that era swept over 
the axis of the scarcely emergent Rocky Mountains. Into 
them, from the erosion of the non-resisting strata of their mar- 
gins and encircling ridges—an erosion caused by heavy rain- 
falls which appear at times to have acquired the strength and 
permanency of the precipitation in the tropical rain areas— 
was washed enormous quantities of shore sand and continental 
mud and silt. These contributions of earthy matter in con- 
junction with the organic testaceous life of the lakes were 
finally consolidated into deposits of shales, marls and earthy 
limestones. Into the wide bosom of these contiguous and 
more or less connected sheets of inland water was also gathered 
the remains of a remarkable fauna, wherein, as Dr. Newberry 
remarks, we have the proof “that during unnumbered ages 
this portion of the continent exhibited a diversified and beau- 
tiful surface, which sustained a luxuriant growth of vegeta- 
tion and an amount of animal life far in excess of what it has 
done in modern times.” The fossilization of these mammals 
(Carnivora, Insectivora, Ruminantia, Pachyderma, Rodentia) 
is a matter of considerable interest. They must have been in- 
troduced into the lakes by sudden meteorological emergencies 
when their own capture and imprisonment in these seas was 
synchronous with violent terrestrial denudation by which they 
_ were safely entombed. Their own character and that of the 
associated flora forbid the supposition, advanced by Lyell and 
apparently applied by Hayden, as to their having fallen into 
the waters through breaking ice, over whose precarious sur- 
*It must be remembered, however, as Dr. Hayden indicated, that ‘‘ the lowest ; 
beds of the Tertiary exhibit a somewhat brackish or estuarine character, and & 
few fossils (Ostrea subtrigonalis) are found which are peculiar to such waters.” 
Preface to Leidy’s Extinct Mammalian Fauna. 
