ORIGIN OF FOLDED MOUNTAINS — PROUTY 297 



pressure all substances would be considerably reduced in volume, 

 and it is not unlikely that there is in the earth a gradual molecular 

 change of the lighter minerals into the heavier minerals, thus caus- 

 ing earth shrinkage. This molecular change is known to have oc- 

 curred in the once more deeply buried portion of the visible crust 

 With release of pressure in surface layers of the earth, minerals 

 tend to form with less density. 



8. The amount of yielding of the earth in response to load is much 

 too great to be explained by the elasticity of the rock. It can best 

 be explained by the settling of the crystalline surface portion of the 

 earth into the glass}', solid-fluid deeper portion, as a mass of soft, 

 though elastic, beeswax will hold its shape and settle into a much 

 harder, though solid-fluid pitch. We may think of this deep-lying, 

 glassy, solid-fluid mass as the ocean on which ride the more rigid and 

 elastic, crystalline rock-ships of the continents. When heavy load- 

 ing occurs, a ship sinks deep in the water, and when unloaded, it 

 displaces less water and rises. The geologist speaks of this com- 

 pensation for load as isostatic adjustment. The glassy basaltic 

 layer is highly sensitive to sudden transverse earthquake waves, 

 thus appearing as an elastic body, but at the same time it yields 

 readily to small, slowly applied forces. 



In discussing the origin of folded mountains we must bear in 

 mind that a number of the above conclusions concerning the earth's 

 interior are relatively ne.w and not fully proved. 



Since most folded mountains are near the borders of continents 

 and since most of them are made up in large part of great thick- 

 nesses of sedimentary rock, deposited in shallow water, it is ap- 

 parent that the mountains have developed where once there was a 

 slowdy sinking basin of marine sediments (a geosyncline). In most 

 theories of origin, therefore, the geosyncline is the controlling factor 

 of location. Any theory to explain folded mountains nuist, of 

 course, meet all the conditions of location, extent, characters of 

 folding, amount of earth shortening, and sources of materials. 



In considering present-day theories let us confine ourselves to 

 those most generally accepted. 



SINKING GEOSYNCLINES 



One of the older theories is the so-called " geosynclinal theory '*, 

 which holds that in regions of extensive sedimentation, usually along 

 the border of continents, the slowly sinking geosynclines carry the 

 unconsolidated and water-soaked sediments to great depths. These 

 sediments become greatly heated by the ascending isogeotherms. 

 With the increase in temperature and probable partial fusion in 

 depth comes increase in volume, which finally more than offsets 

 the slow sinking. This zone of highly heated and relatively weak 



