202 FORMATIONS OF THE 
the upper layers of sedimentary deposits must always be the newest and last to 
have settled down. 
These bone and shell beds constitute what is now known as the Tertiary or 
Cainozoic grand division of the fossiliferous rocks, and overlies the chalk of Eng- 
land and the cotemporaneous marly limestones and argillaceous beds of this coun- 
try; and, with the exception of transported, superficial sands, gravel, erratics, 
marls, and alluvial earths, are, unquestionably, the most recent of the sedimentary 
strata. These tertiary rocks are of great thickness, and admit of being subdivided 
into subordinate groups and members, of older and newer dates—chronologi- 
cally, as well as paleontologically, distinctly separable from each other: thus 
we have become acquainted with a lower, a middle, and an upper group, and even 
subdivisions of these groups, in each of which peculiar and distinct races of ani- 
mals are found. By these, any given member can at once be identified, even on 
remote continents. For instance, the gigantic animal, the skeleton of which was 
discovered in the Bad Lands, called the Puleotheriwm, characterizes the lowest 
group of the formation: its remains are confined exclusively to these eocene beds, 
both in Europe and this country ; whereby we learn that this animal lived during 
the dawn of that geological epoch, and became entirely extinct before the middle 
group began to accumulate, which latter does not contain a vestige of their bones, 
though rich in the remains of an entirely different set of extinct animals. The 
same is true of the uppermost and most modern beds of the formation, as com- 
pared with the middle and lowest divisions. 
Now it is an axiom in Geology, which all experience fully confirms, that there 
never is any reversal of superposition,—these tertiary beds invariably occupying 
the same relative position with respect to the chalk formation; being always 
above it—never below it, so long as they remain in their original undisturbed con- 
dition. They may be twisted, contorted, and sometimes even turned and folded 
under the upper, over limited spaces; but these are local inversions of the order of 
arrangement by subsequent disturbance, and occur only in mountain chains, in 
which powerful subterranean forces have been at work; and close observation can 
even there, in many instances, trace the continuity of strata around the axis of the 
plicated, subverted beds. 
In such situations, the strata may be baked, indurated, and greatly altered from 
their original appearance, but all this does not, by any means, militate against the 
general proposition. Wherever organic remains can in such cases be detected, they 
always prove to be infallible guides to unravel the complicated structure, and 
solve the difficult geological problems which such regions frequently present. 
Another self-evident fact of this science regards all strata which have been rent 
asunder, broken, tilted, or otherwise disturbed, as, in every case, more ancient than 
the dislocating forces and eruptions producing such derangement of the bed; and 
older, also, than the rocks which, in a nascent state, may be thrust up through the 
fissures and parted walls of the superincumbent layers. 
Admitting these facts, the corollary follows, which determines the age of moun- 
tain chains; and which may now be illustrated by demonstrating the period of the 
