.12 Classification of Rocks. 



other. A '^formation" consists of any group of rocks which can be dis- 

 tinguished from all other groups by some particular mark. The thickness 

 of these groups varies from a few feet up to several thousand. Thus the 

 Potsdam sandstone, hereafter to be mentioned, has a depth of only about 

 250 feet, while the Hudson River group is at least 1,000. The formations 

 are deposited one above the other in regular sheets in the order in which 

 they were accumulated upon the bottom of the sea. In the great basin of 

 sedimentary rocks of which we have made mention as covering so large a 

 portion of North America, this sandstone forms the lowest of those stone 

 leaves. It rests immediately upon the bottom of one of the primeval seas, 

 and the other formations repose upon it like so many sheets of paper, each 

 containing certain fossil forms peculiar to itself and not found in any of 

 the others. 



Geologists find at the bottom, certain rocks which are not stratified, and 

 which do not contain fossils, these all appear to have been once in a state of 

 fusion, they constitute what may, for our present purpose, be supposed to 

 have been the original surface of the earth. In this original surface there 

 appear to have been certain great cavities, corresponding in size to those 

 occupied by the oceans of the present day. There evidently was a time 

 when the fii'st waters filled those wide and deep gulfs formed to receive them^ 

 and we have reason to believe that immediately after this event the filling 

 up of the first oceans with water, commenced the process of forming the first, 

 the lowest, and oldest stratified rock. AVe cannot say that this latter has 

 yet been discovered. The progress made in the researches of Geologists 

 after the oldest of the stratified rocks has ever been retrograde, that is, 

 a certain set of strata, may be to-day considered the most ancient, but the 

 explorations of to-morrow may shew, that in another place still older layers 

 exist beneath these. From the surface downwards for a distance of about 

 ten miles, all the formations have been examined and marshalled into an 

 order at present pretty accurately ascertained. 



The following is the most recent classification of Sir Charles Lyell : — 



CLASSIFICATION OF THE FORMATIONS. 



A. POST-PLIOCEXE. 



The Post-pliocene is thus divided : — 1. Recent consisting of the Peat 

 mosses of Great Britain and Ireland, Avith the shell marl containing human 

 remains and works of art. The deposits accumulating on the bottoms of 

 the existing lakes and seas belong to this division. 2. Post-pliocene. — All 

 the shells found in this formation are of existing species, but there are no 

 human remains ; and of the quadrupeds, Avhose bones have been found, part 

 are of extinct species. It appears that the clay, sand, and gravel of ike 

 valleys of the St. Latvrence and Ottawa containing sea shells, or the 

 skeletons of marine fish, are to be referred to the Post-pliocene. The 

 above groups are also called Post Tertiary. 



B. Pliocene. 



» 



The Pliocene is thus divided : — 3. Newer Pliocene or Pleistocene. — 



