302 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1^33 



ing, especially of subocean crust and continental borders, with the 

 outpouring of much lava. 



Since the continents are in isostatic equilibrium (floating in a sea of 

 lava), with decreased lava density they would stand relatively less 

 high above the sea than formerly, provided the lava could escape, and 

 we would have a period of ocean-basin spread and shallow sea trans- 

 gression. With such expansion, the circumference would be, accord- 

 ing to Joly, between 30 and 40 miles longer than normal. 



When crystallization and shrinkage begin, as a result of cooling, 

 the geosynclines are folded and pushed deeper into the earth. With 

 continued crystallization and differential settling, the ocean basins are 

 depressed more than the lands and are underthrust against the 

 continental borders. With the return of the magma to a denser con- 

 dition, the greatly thickened zone of relatively light sedimentary rock 

 in the geosyncline is slowly elevated to a lofty mountain range, while 

 the continents are considerably elevated. According to this theory 

 the folding occurs during the late molten and early crystalline stages 

 and most of the elevation comes later. 



During the molten stage the tidal and precessional forces cause the 

 crustal masses to have a slow migration westward, thus allowing the 

 highly heated subcontinental magma to be carried into the ocean basin 

 segment, and the continents to come to rest over the somewhat cooler 

 subocean portions. It is thought that this relatively westward drift 

 of the crust accounts for the great outpouring of basalts usually on 

 the western side of the continental masses, as that of the Columbia 

 River Plateau of western North America, the Deccan Plateau of 

 western India, the Hebridean area west of Europe, and the Disco 

 Flow west of the mountains of Greenland. By this same force, the 

 continent with the deepest keel and the greatest frictional resistance 

 to drift would have the least westward displacement. Asia, with the 

 highest mountains, and therefore the deepest keel, should in the next 

 molten period migrate less than the rest, as indeed it may have done 

 in the former molten periods. The fact that South America has a 

 deeper keel than North America would also explain its less westward 

 migration. 



According to the Joly theory the earth is now in the solid crustal 

 stage, with continental growth not yet completed. 



This theory has the advantage over all others of logically explain- 

 ing the periodic flooding and elevation of the continents. It also 

 successfully explains the folding of the mountains and the igneous 

 activity associated with the early part of mountain building. It 

 also supplies a logical explanation for the elevation of the mountains, 

 long after their folding. 



If the continents are shifted to the west during the hypothetical 

 molten stage there should be an accumulation of acid rock material. 



