CHAPTEE V. 



The Gbologioaii and Geogeaphicaii EELATioifs op Bieds. 



T^KT] 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 pseudo- 

 morpTis, 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, e. g., as footprints 

 — made upon it ; or (4) Casts, which may be casts of moulds or 

 casts of hoUow structures, such, e. g., as a cast of the cavity of 

 the skuU. 



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 Palceozoic. 

 The second or middle group of strata is called Secondary or 

 Mesozoic. The uppermost and least ancient group consists of 

 strata called Tertiary or Gainozoic, upon the uppermost surface 

 of which the " recent deposits " (which are but their modem 

 continuation) lie. 



Each of these three great groups is made up of a certain 

 number of subordinate groups of strata, or " formations." 

 Thus the " Palaeozoic " rocks are made up of the Laurentian, 

 Cambrian, Silurian, Devonian, Old Bed Sandstone, Carboniferous, 

 and Permian formations. The "Mesozoic" rocks are made up 



