322 HISTORY OF GEOLOGY AND PALEONTOLOGY. 



towards the lines of least resistance in the mountain-system, 

 namely, the anticlinal axes of the folds and arches. Thus they 

 accentuate the appearance of upheaval at the surface, and form 

 the axes of the highest chains, which as a rule consist of ancient 

 crystalline rocks. 



But as the origin of a mountain-system occupies long geo- 

 logical epochs, many changes of temperature may take place 

 in the subterranean masses. Every rise of temperature causes 

 a new movement of expansion, and the mountain-chains may 

 rise higher and higher above the surrounding areas. Fissures 

 and faults are phenomena of contraction produced by cooling, 

 and are therefore usually younger than the folding and upheaval 

 of the mountain-chains. With every crust-rupture a subsidence 

 of one or both sides of the fissure is commonly associated. 



Mellard Reade cites examples chiefly from British and North 

 American geological literature in support of his theory. The 

 weakness of the theory consists in its treatment of mountain- 

 making as a merely local phenomenon ; it assumes rather than 

 explains that the expansion of limited crust-blocks by little and 

 little can effect the uprise of vast mountainous tracts. 



The American geologist, C. E. Button, in a paper " On 

 some of the Greater Problems of Physical Geology," in 1892 

 also contests the Contraction Theory and proposes his theory 

 of " Isostasy." He points out that the earth's crust is not 

 homogeneous, but consists of heavier and lighter masses ; the 

 effort to arrive at equilibrium causes the heavier masses to 

 subside and the lighter masses to rise as crust-buckles. If an 

 area which has already subsided is weighted by thick masses 

 of sediment it must sink farther, and if simultaneously the 

 adjacent crust-buckle is lowered by the agencies of surface 

 denudation, the socket of the arch is so much lightened and 

 rises farther. Should these movements overcome the rigidity 

 of the earth's crust, Button supposes that in the littoral sedi- 

 ments, crust-creep or flow takes place towards the continent in 

 course of denudation, and this flow movement may become 

 so intense as to produce folds and build up mountain-chains. 



Br. Reyer, another opponent of the Contraction Theory, 

 has suggested a theory of mountain-making based upon exten- 

 sive crust-slip. He assumes that every system of crust-folds 

 begins with a crust-rupture and with the sinking of several 

 crust-blocks towards one direction, so that the earth's relief is 

 made unsymmetrical, with a definite slope on one side. If 



