‘ 
Features of the Earth’s Surface. 463 
mistakable structural evidence of such crushing, except in the 
case of mountain chains, I have preferred to attribute the for- 
mation of continents and sea bottoms to unequal radial con- 
traction. 
I wish next to show that this theory of mountain chains 
explains in a satisfactory manner not only the mountain eleva- 
tion and the phenomenon of plication and slaty cleavage, but 
also all the most conspicuous phenomena of mountain chains an 
of igneous agencies. The satisfactory explanation of these 
become, of course, strong evidence of the truth of the theory. 
The further development of the theory will be best undertaken 
in connection with the explanation of these phenomena. 
(A.) Thick sediments of mountain chains. It is a well-known 
fact, first brought prominently forward by Prof. Hall, that 
mountain chains are composed of enormous masses of sediments. 
This fact forms the basis of Hall’s sedimentary theory. Prof. 
Whitney,* it is true, thinks that the sedimentary theorists have 
mistaken cause for effect,—that thick sediments are not the 
cause of mountains, but mountain chains are the cause of thick 
sediments. He believes that a granite axis upheaved out of 
the sea has furnished by erosion the sediments which have been 
deposited on their flanks. But when we remember the immense 
thickness of these sediments and their extent, and the com- 
parative narrowness of the granite axis which furnished their 
materials, we may well ask what must have been the original 
altitude of this granite axis! It seems impossible that the 
immense mass of sediments invulved jin the structure of the 
whole chain. Not only so, but in many chains the strata are 
nite axes. My own belief is that all, smaller and greater, 
ve been fore ) 
I believe, are not the débris of the granite axis of the chain; 
position more definitely: Mountain chains are 
ttoms 
where immense thickness of sediments have accumulated ; and as 
the greatest accumulations usually take place off the shores of con- 
tinents, mountains are usually formed by the up-pressing of mar- 
ginal ns. We will make this plainer by some illus- 
* Mountain Building, &c., pp. 102 and 103. 
