§ 9- CLASSIFICATION OF ROCKS AND STRATA. 37 



The superficial crust of the globe is composed of nume- 

 rous layers and masses of earthy substances, of which 

 various combinations of iron, lime, and silex or flint, 

 constitute a very large proportion ; the latter forming 

 forty-five per cent, of the whole. Those strata which have 

 been deposited the latest, bear evident marks of mechanical 

 origin, and are the water-worn ruins of older rocks ; as we 

 penetrate deeper, deposits of a denser character appear, 

 which also exhibit proofs of having been formed by the 

 action of water; but when we arrive at the lowermost 

 accessible to observation, a crystalline structure generally 

 prevails; and while in the upper stratified rocks, the 

 remains of animals and vegetables are found in profusion, 

 in the ancient crystalline masses all traces of organic forms 

 are absent. 



9. Classification of rocks and strata. — In the infancy 

 of the science these remarkable facts gave rise to an ingenious 

 theory, which being founded on insufficient data, has proved 

 untenable. It will, however, be requisite briefly to explain 

 this hypothesis, as some of the terms still employed in geo- 

 logical nomenclature refer to the speculations it suggested. 

 According to this theory the mineral masses composing the 

 earth's crust, are separable into three grand divisions or 

 groups, as follow : — 



1st. Primitive Mocks, — The Primitive (now termed 

 Primary) rocks, are of a crystalline structure, and have 

 evidently been brought to their present state by igneous 

 agency; they comprise granite, sienite, porphyry, &c. 

 These are the lowermost rocks, and constitute the founda- 

 tion on which all the newer strata have been deposited ; 

 they also attain the highest elevations on the surface of the 

 globe. They were termed primitive, because, from the 

 entire absence of organic remains, it was inferred that they 

 were formed before the creation of animals and vegetables; 

 but it is now certain that granite and its associated rocks 



