198 SEE— FURTHER RESEARCHES ON [April 24, 



except ill the layer just beneath the crust. These superficial move- 

 ments are called earthquakes, and are caused chiefly by the leakage 

 of the oceans. Observations show that the depth of such disturb- 

 ances in all cases is less than 40 miles. This accords with the 

 theory of isostacy, and confirms the conclusions drawn from that 

 theory that all surface inequalities are compensated for at but a 

 slight depth. 



§21. Uplifts along the Andes sho7c that the mountains arc not 

 sinking under their own zveight. — In Professor Sir G. H. Darwin's 

 paper on the stresses in the earth, above cited, he has also con- 

 sidered harmonics of high order, corresponding to the case of a 

 series of parallel mountains and valleys, which thus corrugate a 

 mean level surface with an infinite series of parallel ridges and 

 furrows. Here the stress-difiference depends only on the depth be- 

 low the surface, and is independent of the position of the point 

 considered with respect to ridge and furrow. Taking a series of 

 mountains 13,000 feet (about 4.000 meters) above the valley bottoms, 

 formed of granite of density 2.8, he shows that the maximum 

 stress-difiference is 4 X 10^ grammes weight per square centimeter 

 (about the tenacity of cast tin). And when the mountain chains are 

 314 kilometers apart, making the ridges about 78 times wider than 

 they are deep, the maximum stress-difiference is reached at a depth 

 of 50 kilometers below the surface, or at a depth of 12^ times the 

 height of the mountains above the valleys. Thus for mountains of 

 the height of our average ocean depth, the maximum tendency to 

 flow would be at a depth of about 31 miles. (Cf. " Nat. Phil," 

 Vol. I, Part II, §832.) 



If earthquake shocks were due to such flowage the mountains 

 would be gradually reduced in height. Instead of this settling oc- 

 curring, mountains like the Andes are still rising, as we may infer 

 from the fact that after an earthquake the adjacent sea coast often 

 is elevated and higher than before; while the sinking of the adjacent 

 sea bottom, indicated by the accompanying seismic sea wave, shows 

 that the bed of the sea was undermined by the expulsion of the 

 material pushed under the land and mountains. This state of fact 

 eniphatically contradicts the viezv that these great seismic disturb- 

 ances are due to the flowage beneath the crust arising from the 



