410 Theories oj the Formation of Mourdains, 



ated from each other by valleys of variable width and depth. The 

 height of these hills may be stated in a general way as ranging 

 from five hundred to five thousand feet above the level of the sea. 

 They attain their greatest developement in the States of Pennsyl- 

 vania, Virginia, North Carolina and Tennessee where they have 

 been most successfully studied by the brothers Professors W. B. 

 and H. D. Rogers, The results of the observations of these two 

 eminent geologists are given in a masterly paper read before the 

 American Association for the Advancement of Science in 1842 

 and recently in a more matured form by H. D. Rogers in his 

 magnificent work on the geology of Pennsylvania. 



According to Professor Rogers the Appalachians consist nearly 

 altogether of stratified rocks of palaeozoic age including all the 

 American formation from the base of the Silurian up to the top 

 of the carboniferous. These rocks were deposited in nearly hori- 

 zontal strata on a sea bottom, which in the region now occupied 

 by the mountains in question, kept constantly subsiding during 

 the whole period of their accumulation. South-east there existed 

 in the place of the present Atlantic Ocean a vast continent from 

 the waste of whose shores the material out of which the strata 

 were formed was derived. In consequence of their proximity to 

 the shore the formations on the south-east accumulated more 

 rapidly than they did towards the south-west. For this reason 

 there is now found a much greater thickness of the same rocks 

 in Pennsylvania, Virginia, and other north eastern States than in 

 those which lie further west such as Ohio, Indiana, Illinois and 

 Iowa. The strata remained in their nearly horizontal position, 

 perhaps sloping gently towards the south-west, until the close 

 of the Carboniferous era. Then by some great disturbance of the 

 eauinbrium of the forces of nature they were thrown into a series 

 of vast wave like undulations. The profile of these waves imme- 

 diately after their elevation must have been somewhat similar to 

 the following figure. 



Fig. I. 



Fig. 1. — Ideal section across the Appalachiang. The dotted space is 

 intended to represent the rock of the original bottom of the ocean. The 

 black undulated line represents the stratiiBed rocks after having beeu 



