54 OBSERVATIONS ON THE CAUSES OF ELEVATION, ETC. 



by some of the American geologists who have explored the 

 western territories of America. These have pointed " in proof 

 of its truth to evidence of continuous subsidence in tracts 

 where there was prolonged deposition and of the uprise and 

 curvature of originally horizontal strata over mountain ranges 

 like the Uintah Mountains in Wyoming and Utah, which have 

 for a long time been out of water." Dr. Greikie, in commenting 

 upon this theory, admits that in so far as the internal structure 

 of rocks may be modified by such progressive increase of 

 temperature as would arise from superficial deposit, the cause 

 of change must have a place in geological dynamics, but he 

 cannot allow that the removal and deposit of a few thousand 

 feet of rock should exert such an influence as -to affect the 

 equilibrium of the crust ; for to admit this, '* would evince such 

 mobility in the earth as could not fail to manifest itself in a 

 far more powerful way under lunar and solar attraction." He, 

 however, goes on to say " that there has always been the closest 

 relation between upheaval and denudation on the one hand and 

 subsidence and deposition on the other, is undoubtedly true." 

 But he adds the significant words that " denudation has been 

 one of the consequences of upheaval, and deposition has only 

 been kept up by continual subsidence." Certain questions 

 bearing upon the permanence of continental areas and great 

 oceanic basins are involved to a great extent in the views under 

 discussion, and may have led to the adoption of restricted views 

 as to fundamental laws of causation. 



Prof. C. Loyd Morgan in a recent article on elevation and 

 subsidence (Geol. Mag., July 1888) in criticising the views of 

 Mr. Mellard Eeade and others as regards the effects alleged 

 to arise out of the transfer of sediments suggests other ways 

 in which the loading and unloading of the earth's crust may 

 indirectly bring about subsidence and elevation, and at the 

 same time ingeniously advances reasons in favour of the 

 existence of an underlying liquid or viscous substratum. Ac- 

 cordingly, without committing himself to the acceptance of the 

 theory held by those who attribute subsidence to mere weight 

 he suggests to the upholders of that theory that the added 

 weight of the sediment above would entail on this hypothesis 

 an added weight below — that is if we suppose that the solidi- 

 fied rock adheres to the lower surface of the crust region. In 

 a region undergoing denudation, on the other hand, the 

 lightening of the load would entail the melting of some of the 

 solidified or crystallised magma, assuming with Mr. Mellard 

 Reade and Mr. Davison, that owing to the cooling and con- 

 traction of the earth's crust there is at some depth beneath the 

 surface a level of no stress, where there is neither lateral 

 compression nor extension, though the rocks are subject 

 to the vertical pressure of the overload. He then sug- 

 gests that throughout the zone of maximum tension 



