616 PALMONTOLOGICAL GEOLOGY. [Book V. 
Paleotherium, Anoplotherium, Hyopotamus, and Anthracotherium of 
the older Tertiary formations; Mastodon, Elephant, Hyzna, Cervus, 
and Equus of younger Tertiary formations. The occurrence of such 
organisms in any rock at once decides the great division of geological 
time to which the rock must be assigned. 
The type fossils of a formation, after sufficiently prolonged and 
extended experience, having been ascertained, serve to identify that — 
formation in its progress across a country. Thus, as we trace the 
formation into tracts where it would be impossible to determine 
the true order of superposition, owing to the want of sections, or to 
the disturbed condition of the rocks, we can employ the type-fossils — 
as a means of identification, and speak with confidence as to the suc- 
cession of the rocks. We may even demonstrate that in some 
mountainous ground the beds have been turned completely upside 
down, if we can show that the fossils in what are now the uppermost 
strata ought properly to lie underneath those in the beds below them. 
Prolonged study of the succession of organic types in the geo- 
logical past all over the world, has given paleontologists some con- 
fidence in fixing the relative age even of fossils belonging to previously 
unknown species or genera, and occurring under circumstances where 
no order of superposition can be found. For instance, the general 
sequence of mammalian types having been fixed by the law of super- 
position, the horizon of a mammaliferous deposit may be approxi- 
mately determined by the grade or degree of evolution denoted by its 
mammalian fossils. Thus, should remains be generically abundant, 
differing from those now living and presenting none of the extreme — 
contrasts which are now found among our higher animals, should 
they embrace neither true ruminants, nor solipedes, nor probos- 
cidians, nor apes, they might with high probability be referred to 
the Eocene period. Reasoning of this kind must be based, however, 
upon a wide basis of evidence, seeing that the progress of develop- 
ment has been far from equal in all ranks of the animal world. 
Observations made over a large part of the surface of the globe 
have enabled geologists to divide the stratified part of the earth’s 
crust into systems, formations, and groups or series. These sub- 
divisions are frequently marked off from each other by lithological 
characters. But, as already remarked, mere lithological differences 
afford at the best but a limited and local ground of separation. 
Two masses of sandstone, for example, having exactly the same 
general external and internal characters, may belong to very 
different geological periods. On the other hand, a series of limestones 
in one locality may be the exact chronological equivalent of a set 
of sandstones and conglomerates at another, and of a series of shales 
and clays at a third. 
Some clue is accordingly needed which will permit the divisions 
of the stratified rocks to be grouped and compared chronologically. 
This fortunately is well supplied by their characteristic fossils, 
' Gaudry, “ Les enchainements du Monde Animal,” 1878, p. 246, 
