SUBMARINE MOUNTAINS 97 



i. The chemical composition, structure, and magnitude of 

 the volcanic islands, as well as the initial temperature of their 

 lavas seem to find best explanation if the corresponding vents 

 are assumed to have been fed from a simatic substratum, 

 world-circling and so hot that it is necessarily in the glassy 

 state. 



2. The volcanic islands, like many shoals, represent extra 

 masses of rock, built up on the floor of the ocean in the form 

 of high, bulky mountains, whose relief is only partly, if at all, 

 "compensated" by abnormal density for the rock beneath the 

 upstanding masses. 



3. Although many of the volcanic loads are great and have 

 been weighing on the old sea floor for millions of years, they 

 are to all appearances stably borne; they seem not to be now 

 bending down or breaking down the earth's crust. 



4. Computation, founded on laboratory experiments on the 

 strength of rock at high, all-sided pressure, makes it highly 

 probable that the volcanic loads can be stably carried by a true 

 crust with thickness of about 50 miles. 



5. The ocean deeps represent negative loads of the same 

 order numerically as that of the positive, volcanic loads, and 

 here again the crust unaided seems competent to give full 

 support. 



6. Because the crust is so sturdy, neither kind of loading 

 gives a valid argument against extreme weakness for the sub- 

 stratum. 



7. Such weakness appears to be an inescapable condition for 

 the development of mountain chains of the Alpine type. The 

 mountain structure and the characteristically arcuate ground- 

 plan of a chain prove that crust-blocks of continental dimen- 

 sions have moved horizontally over the earth's body, through 

 distances of scores of miles, if not hundreds of miles. The 

 horizontal displacements defy understanding unless there be 



