MOVEMENTS AND DEFORMATIONS OF THE EARTH'S BODY. 585 



to considerable depths. For example, if the hypothetical level of no 

 stress is eight miles below the surface, as computed on certain assump- 

 tions, the concave portion must be so broad that the isogeotherms will 

 also be concave outAvard at something near that depth; in other words^ 

 the main part of the zone of thrust must be concave. A narrow con- 

 cavity at the surface, such as an ordinary valley in a portion of the 

 crust that has the average convexity, would not seriously depress the 

 isogeotherms, or affect the zone of thrust, but a valley several times 

 eight miles (level of no stress) in breadth would. For inspecting the sur- 

 face of the earth in this regard, it is convenient to know what amounts 

 of fall below the level surface give a true plane for given distances. 

 These are sho-^n in the following table : ^ 





Length of normal to chord at 



Average fall of 



Length of arc 

 in miles. 



middle point in 



true plane from 

 level plane per 







iXL-ic , III itrci . 



Greater fall 





Feet. 



Fathoms. 



gives concavity. 



25 



100.3 



16.7 



8 



50 



432. 



72. 



17.3 



75 



913.4 



152.2 



24.3 



100 



1,684. 



280.7 



33.7 



150 



3,748.8 



624.8 



49.9 



200 



6,674 . 



1,112.3 



66.7 



250 



10,369.9 



1,728.3 



82.9 



300 



14,942. 



2,490.3 



99.6 



400 



26,664 . 



4,444. 



133.3 



500 



41,659. 



6,943. 



166.6 



Applying these criteria to the surface of the lithosphere, it is found 

 that coiwave tracts from 100 to 300 miles in breadth are not uncorjimon. 

 The more notable of these are sho^Mi in black on the accompanying 

 map. Fig. 454, and two typical ones are sho^Ti in cross-section in Figs. 

 455 and 456. It is to be observed that concave tracts border the con- 

 tinents YeiY generally. They are connected with the descent from the 

 continental shelf to the abysmal basins, and are uns;yTnmetricaI Not- 

 able conca\ities are found in some of the great valleys on the conti- 

 nental platforms. The basins of Lake Superior, Michigan, Huron, 

 and Ontario are in part concave; so are Puget Somid, the Adriatic, 

 and the Dead Sea; so also are the valleys of California, of the Po, and 

 of the Ganges, when the adjacent mountains are included. Some of 



^ Prepared at the authors' request by W. H. Enimons. j 



