WHAT IS TERRA FIRMA?— A REVIEW OF CURRENT 

 RESEARCH IN ISOSTASY. 



[With 3 plates.] 



By Bailey Willis. 



What are the foundations of the earth? On what do mountains, 

 continents, and ocean basins rest? When men build they look to it 

 that the foundations are firm enough to support the weight of the 

 structure, or the building crushes its foundation and falls. Are there 

 any rocks firm enough to bear the weight of mountains or continents 

 without crushing? 



The crushing strength of rocks, as ascertained in a testing ma- 

 chine, varies from 8,000 to 20,000 pounds to the square inch, and 

 their average density is such that the weight of a column 3 to 5 

 miles high would crush its base. But among mountains there are 

 many that are more than 3 miles high and some that exceed 5 miles. 

 Their pyramidal form aids that portion of the foundation which is 

 beneath the high peaks, but it has nevertheless been observed in tun- 

 neling that the rocks are in a state of great strain, as was the case, 

 for instance, with the granite penetrated by the Simplon Tunnel 

 beneath the Alps. 



In the case of a plateau the form is that of a block, and where the 

 height exceeds 3 miles the base probably approaches a crushed con- 

 dition. Tibet thus stands above the general level of the Asiatic 

 Continent. Asia itself may be described as a plateau, having an un- 

 even surface, but rising on the average 3 to 4 miles above the bottoms 

 of the ocean basins. Considered, then, as a mass whose base is on a 

 level with the depths of the oceans, Asia is so high that its weight 

 must exceed the load which can be supported by rocks, as we know 

 them. The same is true of other continents. 



Thus it seems reasonable to think that the foundations or rocks 

 beneath the continents may approach a crushed condition or may 

 actually be crushed. 



Our thought has passed from mountains to plateaus and to conti- 

 nents. The foundations of continents comprise one-fourth of the 



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