Kansomk.] 



The Great I 'alley. 



417 



Again, as in that valley, these are not the conditions that would be 

 brought about by ideal isostasy. The great alluvial fan of the 

 Colorado has been built up above sea-level faster than subsidence 

 (if there be any actual subsidence) could carry it down, while, fur- 

 ther south, the Gulf of California, which, down to the islands of 

 Tiburon and Angel de la Guardia, is a portion of the continental 

 platform, with no depths greater than 200 fathoms* testifies to a 

 depression that is in excess of any available load upon its floor. 

 Thus in the north, the subsidence has been too small, and in the 

 south, too great to be accounted for by the theory of isostatic 

 adjustment to sedimentary loading, and must have been independent 

 of it. 



GENERAL DISCUSSION OF THE THEORY OF ISOSTASY. 



The Data upon Which the Theory Has Been Made to Rest. — 

 Undoubtedly the first suggestion of the theory of isostasy came 

 from the observation that, as a general rule, the thicker masses of 

 sediments laid down during the past geological ages have been 

 deposited over subsiding areas, and at such a rate that subsidence 

 and deposition have kept pace with each other. Thus accumula- 

 tions many thousands of feet in thickness, which have been brought 

 into view by the ever-recurring cycles of earth movement, reveal 

 the fact that at no time during their deposition were they covered 

 by any great depth of water. On the contrary, at frequent intervals, 

 from the top to the bottom of such a series, we may see evidence 

 that passing showers pitted the mud left bare by the tide, or that 

 the animals of those times recorded their tracks on its surface. 



The obvious and simple explanation of such conditions of sedi- 

 mentation was found in the isostatic theory, which by making the 

 load the cause of the subsidence, appeared to insure the delicate 

 balance between deposition and sinking which the facts seemed to 

 demand. But, simple as the explanation at first appears, it leads 

 ultimately to greater complexity, by introducing a distinct class of 

 earth movements, and by demanding a mobility in the earth's crust 

 which seems to be opposed to astronomical and physical calcula- 

 tions, and to many geological facts. 



* Emmons and Merrill, loc. cit., p. 513. 



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