292 ANNUAL REPORT SMITHSONIAN INSTITUTION, 192 8 



the different oceans and the different parts of the same ocean show 

 similar ranges in depth. 



Future detailed investigations of selected parts of the ocean floors 

 may bring strong evidence to bear for or against the hypothesis of 

 fragmentation, but our present knowledge on this subject is in- 

 definite. The arguments here set forth merely show that frag- ■ 

 mentation of parts of continents is not disproved by what is now 

 known as to the relief of the ocean floor. 



Let us turn, then, to the third argument, tliat dealing with change 

 of density and resulting isostatic movements as the immediate cause 

 of fragmentation. On the continents such changes in density clearly 

 seem to have taken place, but the evidence is best where the change 

 is toward a lesser density and where parts of continents are lifted 

 up into plateaus. As an illustration, most of the great Cordilleran 

 area of North America during the Mesozoic tended to lie not far 

 from sea level. Here ranges of mountains were uplifted from time 

 to time, but their waste was deposited in adjacent regions as sea 

 deposits, or laid down as continental formations at a low elevation. 

 The region of the lofty Colorado plateaus shows, in fact, a strati- 

 graphic section which includes deposits, most of them marine, rang- 

 ing from the envly Paleozoic to the close of the Mesozoic. During 

 all of that vast time, occasional regional subsidence with resulting 

 sedimentation was the rule. No uplift occurred great enough to per- 

 mit erosion to destroy completely the older strata. But during the 

 later Tertiary a reverse movement set in; broad uplift of fault blocks 

 took place so that these masses now constitute plateaus from 6,000 to 

 11,000 feet in elevation. The region is shown by geodetic observa- 

 tions to be in the usual degree of isostatic equilibrium. There would 

 appear to be evidence here, therefore, of a regional decrease in density 

 in late geologic times. The cause of such a decrease would appear 

 to be the rise of molten magmas and the expansion of the cover rocks 

 due to the accession of heat. Vast volumes of magma are needed to 

 account for a change of density in the whole crust sufficient to cause 

 an isostatic uplift of from 1 to 2 miles, and these volumes must there- 

 fore be at such a depth that large local surface outpourings would 

 not be possible as proof of their presence. The Cordilleran belt as a 

 whole, on the other hand, many hundreds of miles wide and running 

 the length of the continent, was the scene of regional igneous activity 

 during the Tertiary era on as large a scale as is loiown anywhere 

 since the Proterozoic. Not all parts of the surface have been covered 

 with extrusive rocks, but the intrusions in depth must be far wider, 

 and for the Cordilleran province as a whole the relationship of 

 igneous activity to the great Tertiary uplift seems clear. 



