the Earth's Contraction from Cooling. 45 



1. Have subsidences been produced by lateral pressure ? 



The theory of Professor James Hall, that the great subsidences 

 of the globe have been made by the gravity of accumulating 

 sediments, has been shown elsewhere* to be wholly at variance 

 with physical law. 



Another theory is presented by Professor LeConte in his 

 recent paper in the last volume of the American Journal of 

 Science, to which the reader is referred. Admitting, with Pro- 

 fessor Hall, that the mean thickness of the accumulations in the 

 Appalachian region of Pennsylvania is 40,000 feet, and therefore 

 that this is the measure of the gradual subsidence that attended 

 their deposition, he shows that the temperature in the bottom 

 deposits would have been, supposing the usual rate of increase 

 downward (1° F. for 58 feet of descent) 800° F., and at 10,000 

 feet 230° F. ; and he argues that hence there would have re- 

 sulted below, first, " lithification and therefore increasing den- 

 sity, and therefore contraction and subsidence pari passu with the 

 deposit ;" next, or at a greater depth, " aqueo-igneous softening " 

 or " melting," the temperature of 800° F. being " certainly suf- 

 ficient to produce this result as well as metamorphism, and 

 during this process the subsidence would probably continue;" 

 and, in addition, the underlying strata on which the sediments 

 were deposited would have participated in the " aqueo-igneous 

 fusion" and thus have added to the result f. 



* Amer. Journ. Sci. 2nd ser. vol. xlii. p. 210 (1866); 3rd ser. vol. v. 

 p. 347 (1873) ; LeConte, ib. 3rd ser. vol, iv. p. 461 (1872). 



f The principal points in Professor Hall's theory of mountains, pub- 

 lished in 1859 (Amer. Journ. Sci. 3rd ser. vol. v. p. 347), are : — 



1. Coast regions the courses of marine currents, and hence of deposited 

 sediments. 



2. The accumulation of sediments by their gravity gradually sink the 

 crust, and thus a great thickness is attained ; the rocks become solidified 

 and sometimes crystallized below. 



3. The continents afterward somehow raised — not the mountain-regions 

 separately. 



4. Shaping of the mountains out of other sediments by denudation. 



5. Metamorphism due to " motion/' " fermentation," and a little heat ; 

 the heat coming up from below (the isogeothermal planes rising) in conse- 

 quence of the increasing accumulations at surface. 



In Professor LeConte's theory (Amer. Journ. Sci. 3rd ser. vol. iv. pp. 

 345,460(1872):— 



1. The same as in Professor Hall's. 



2. As explained in the text above. 



3. After an aqueo-igneous softening of the beds below, the lateral thrust 

 from the earth's contraction pressed together the region of sedimentary 

 accumulation, plicating and crushing the beds. 



4. The elevation of mountains due solely to crushing and plication. 



5. Metamorphism consequent on the heating derived by the rise of the 

 isogeothermal planes. 



