on American Geology, 101 



mountains, the strata which form their base are to be found 

 beneath their foundations at a much lower horizon than in the 

 undisturbed hills of the Mississippi valley, and that to this depres- 

 sion chiefly is due the fact that the mountains of the Appalachian 

 range do not, like those hills, exhibit in their vertical height above 

 the sea the whole accumulated thickness of the palaeozoic strata 

 which lie buried beneath their summits. 



Mr. Hall has made a beautiful application of these views to explain 

 the fact of the height of the Green Mountains over the Laurentides, 

 and of the White Mountains over the former, by remarking that we 

 have successively the Lower and Upper Silurian strata superimposed 

 on those of the Laurentiau system. The same thing is strikingly 

 shown in the fact that the higher mountain chains of the globe 

 are composed of newer formations, and that the summits of the Alps 

 are probably altered sediments of tertiary age. (Am. Jour. Sci, 

 xxix. 118.) 



The lines of mountain elevation of De Beaumont are according 

 to Hall, simply those of original accumulations, which took place 

 along current or shore lines, and have subsequently, by continental 

 elevations, produced mountain chains. " They were not then due 

 to a later action upon the earth's crust, but the course of the chain 

 and the source of the materials were predetermined by forces in 

 operation long anterior to the existence of the mountains or of 

 the continent of which they form a part." p. 86. 



It will be seen from what we have said of Buffon, De Montlosier 

 and Lesley that many of the views of Mr. Hall are not new but 

 old ; it was, however, reserved to him to complete the the- 

 ory and give to the world a rational system of orographic geo- 

 logy. He modestly says, " I believe I have controverted no 

 established fact or principle beyond that of denying the influence 

 of local elevating forces, and the intrusion of ancient or plutonio 

 formations beneath the lines of mountains, as ordinarily understood 

 and advocated. In this I believe I am only going back to the 

 views which were long since entertained by geologists relative to 

 continental elevations." p. 82. 



The nature of the palaeozoic sediments of North America clear- 

 ly shows that they were accumulated during a slow progressive 

 subsidence of the ocean's bed, lasting through the palaeozoic per- 

 iod, and this subsidence, which would be greatest along the line 

 of greatest accumulation, was doubtless, as Mr. Hall considers, con- 

 nected with the transfer of sediment and the variations of local pres- 



