Geology. 233 



''My own ideas are chiefly based on a far more lively concep- 

 tion of this process. I propose to describe the whole movement, 

 above the surface and below it, as the Circulation of the Earth's 

 Crust, because I am supposing this movement to be a universal 

 characteristic of the crust, and the ultimate cause of all the great 

 changes in its surface. I would also regard this Circulation as 

 acting in the most detailed manner between every adjacent ele- 

 vation and depression." 



The author recognizes the improvement of the theory of isos- 

 tasy over other theories of the earth's crust, but he thinks the 

 theory of isostasy is still too incomplete to be of much use to the 

 practical geologist and the object he had in mind is to ''try to 

 advance it a further stage in its development." In this view 

 of isostasy he is correct, for up to this time there have not 

 been many papers dealing with the processes by which isostasy 

 is brought about and maintained. The literature has dealt 

 mostly with the accumulated geodetic data and the results of 

 investigations which have proved that the condition of isostasy 

 exists, at least within the regions covered by the data used. 

 From this point. Col. Tandy discusses Deeps and Rivers, show- 

 ing that he differs from the views generally held by geologists. 



Under the heading River Deltas, he refers to Barrell's use of 

 deltas in his "The Strength of the Earth's Crust" to prove that 

 the earth's crust is able to hold up great loads. Tandy differs 

 from Barrell, though his reasoning is not convincing that he, 

 Tandy, is correct. He simply refers to the fact that the old 

 assumption that the mountains are extra loads has been proved 

 incorrect, and by inference the crust is not strong enough to sup- 

 port a delta. 



Col. Tandy sets forth some views under the heading "Differ- 

 ences of Crustal Temperature" which conform very closely with 

 some of mine contained in two recent papers.^ Each of us 

 worked in entire ignorance of what the other was doing. Tandy 

 calls attention to the views expressed by some geologists that dif- 

 ferences in elevation of the surface of the earth are due to dif- 

 ferences in crustal temperature. He believes that this theory or 

 idea is in material and remarkabe harmony with the main princi- 

 ple of isostasy. He holds that the crust in high regions consists 

 of expanded matter and in low regions of condensed matter. He 

 says, ' ' The varied expansion of the crust, shown by differences 

 of elevation at its surface, may be due to differences of crustal 

 temperature and chemical changes generated by the circulation of 

 the earth 's crusts ' ' 



^ Some Geologic Conclusions from Geodetic Data, Proceedings of National 

 Academy of Sciences, vol. 7, No. 1, pp. 23-28, Jan., 1921. 



The Eelation of Isostasy to Uplift and Subsidence, tMs Journal, vol. 2, 

 July, 1921. 



