CHRONOLOGICAL SUCCESSION. 43 



single example where one may see the Silurian rocks, the 

 Devonian, and the Carboniferous rocks succeeding one another 

 regularly, and in their proper order. This is because the 

 particular region where this occurs was always submerged be- 

 neath the sea while these formations were being deposited. 

 There are, however, many more localities in which one would 

 find the Carboniferous rocks resting unconformably upon the 

 Silurians without the intervention of any strata which could be 

 referred to the Devonian period. This might arise from one of 

 two causes : i. The Silurians might have been elevated above the 

 sea immediately after their deposition, so as to form dry land 

 during the whole of the Devonian period, in which case, of 

 course, no strata of the later age could possibly be deposited in 

 that area. 2. The Devonian might have been deposited upon the 

 Silurian, and then the whole might have been elevated above the 

 sea, and subjected to an amount of denudation sufficient to 

 remove the Devonian strata entirely. In this case, when the 

 land was again submerged, the Carboniferous rocks, or any 

 younger formation, might be deposited directly upon Silurian 

 strata. From one or other of these causes, then, or from subse- 

 quent disturbances and denudations, it happens that we can 

 rarely find many of the primary formations following one 

 another consecutively and in their regular order. 



In no case, however, do we ever find the Devonian resting 

 upon the Carboniferous, or the Silurian rocks reposing on the 

 Devonian. We have therefore, by a comparison of many 

 different areas, an established order of succession of the strati- 

 fied formations, as shawn in the subjoined ideal section of the 

 crust of the earth (fig. 17). 



The main subdivisions of the stratified rocks are known by 

 the following names : 



1. Laurentian. 



2. Cambrian (with Huronian?). 



3. Silurian. 



4. Devonian or Old Red Sandstone. 



5. Carboniferous. 



6. Permian _ 



New Red Sandstone. 



7. Triassic 



8. Jurassic or Oolitic. 



9. Cretaceous. 

 10. Eocene. 



n. Miocene. 



12. Pliocene. 



13. Post-tertiary. 



