Cit SPE: Y: 
Tur GEOLOGICAL AND GEOGRAPHICAL RELATIONS OF BIRDS. 
HE relations which different kinds of Birds and their whole 
class bear to past time is revealed to us by fossil remains 
preserved in the earth’s crust, and by relics found in caves and 
fissures on its surface. ‘ Fossils” may be either: (1) bones 
which retain the greater part of their own mineral matter and 
some of their animal matter also; (2) Substitutes or pseurdlo- 
morphs, which are relics the original substance of which has 
been transformed, particle by particle, into mineral matter— 
ferruginous, calcareous, or siliceous; (3) Moulds, that is, 
a deposit which exhibits impressions—such, ¢. 7., as footprints 
—made upon it; or (4) Casts, which may be casts of moulds or 
casts of hollow structures, such, e. g., as a cast of the cavity of 
the skull. 
The crust of the earth is made up in the first place, super- 
ficially, of accumulations of sands, clays, and gravels, which 
form what are called recent deposits, and are not counted as 
constituting any part of what are spoken of as geological 
strata, which are classified in three great groups, belonging 
respectively to three great epochs. The deepest and most 
ancient group comprises the strata called Primary or Paleozore. 
The second or middle group of strata is called Secondary or 
Mesozoic. The uppermost and least ancient group consists of 
strata called Tertiary or Cainozoic, upon the uppermost surface 
of which the “recent deposits” (which are but their modern 
continuation) lie. 
Each of these three great groups is made up of a certain 
number of subordinate groups of strata, or ‘ formations.” 
Thus the “ Paleozoic” rocks are made up of the Laurentian, 
Cambrian, Silurian, Devonian, Old Red Sandstone, Carboniferous, 
and Permian formations. The ‘* Mesozoic” rocks are made up 
