298 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 3 



sediments, having displaced the stronger rock by isostatic adjust- 

 ment, is in condition to be readily folded by the lateral compression, 

 exerted in the crustal portion of the earth, due to the shrinkage of 

 the earth's interior through loss of heat or otherwise. 



In the early days of this theory shrinking through loss of heat 

 seemed ample to account for the known amount of lateral compres- 

 sion. Later, when larger horizontal movements were recognized, 

 increasing earth density, through recrystallization, was added as 

 another cause of radial earth shortening and compression. It has 

 been apparent, also, that the wedge action of igneous masses, pushed 

 up through the strata and forming great laccoliths in the strata, 

 has been in some regions an added source of lateral compression. 

 This theory is advanced by Arthur Keith, of the United States Geo- 

 logical Survey in accounting for the great amount of lateral com- 

 pression in the Appalachian Mountains. We find that wherever in 

 the Piedmont we have great masses of late Paleozoic igneous rock we 

 have also to the northwest of these areas a marked bulge or salient 

 in the Appalachian folds, as though the introduction of this igneous 

 rock had pushed the folds to the northwest beyond their normal 

 position. Such an igneous mass is to be found extending from 

 Danville, Va., through Greensboro and Charlotte, and on into South 

 Carolina. 



According to this geosynclinal theory all continental borders 

 should show parallel mountain growths. While this is generally 

 true, there are glaring exceptions. For the most part the Arctic and 

 Atlantic Ocean borders are relatively free from such parallel moun- 

 tains. Along the border of these two ocean basins, mountains, where 

 present, approach the shore at such angles as to suggest that they 

 continue beyond the continental border. The Appalachians, for 

 example, pass into the Atlantic from Newfoundland as though they 

 are headed straight for Great Britain. 



In brief, this geosynclinal theory explains the chief cause of zones 

 of crustal weakness and supplies a few of the many possible reasons 

 for crustal buckling. It leaves unexplained many facts connected 

 with folded mountains. 



CONTINENTAL-DRIFT THEORY 



The second theory to be considered is the so-called " continental- 

 drift " theory. It is one of the younger and more spectacular 

 theories, which has a strong appeal to the imagination. If true, it 

 would explain in a simple way many perplexing facts formerly 

 explained in various ways. This fact alone has brought it many 

 defendants. 



According to this theory, in late Mesozoic time all the now sepa- 

 rated continents formed one great continent, Pangaea, in the Greater 



