336 SEE— THE CAUSE OF EARTHQUAKES. [October 19, 



spur/ because the upheaval would then become easy, and more and 

 more lava would be forced into the outlet thus afforded. By means 

 of the earthquakes accompanying the expulsion of lava from the 

 sea valley the crust is broken at various points, and thus the spurs 

 are gradually developed. It is noticeable in all the great chains 

 that these spurs are directed towards the sea. In the Himalayas, 

 for example, they are on the convex side, while in the Sierra Ne- 

 vadas they are on the concave side of the chain ; which shows that 

 the spurs are not due to any process of shrinkage of the earth. How- 

 ever the chains may curve, the spurs will be found towards the sea 

 valleys from which the expulsions of lava have taken place. This 

 arrangement of the spurs shows the process of mountain formation 

 very clearly. 



If we examine the map of Tibet given in the Encyclopedia 

 Britannica, ninth edition, we shall find these spurs mainly on the 

 outside of this great tableland. It was therefore injected from the 

 seas on the north as well as on the south; for the Kuen Lun and 

 Altin Tagh mountains on the north show the spurs almost as dis- 

 tinctly as the Himalayas, on the south; and on the north of Tibet 

 the spurs point towards the Arctic ocean. Tibet was thus upheaved 

 by forces injecting this great tableland on all sides. It is thus a 

 very elevated plateau, enclosed by terribly high mountains. Before 

 the upheaval had attained such great height, no doubt the enormous 

 rainfall produced by these mountains, and hence sinking down in 

 the faults thus opened to the bowels of the earth, contributed greatly 

 to the uplifting forces due to the surrounding seas. In this way one 

 may account easily and naturally for the gradual upheaval of the 

 highest plateau in the world. 



The forces depending on the valleys of the Ganges and Brama- 



putra are still active. And the bones- of elephants and rhinoceroses 



now found 15,000 feet above the sea, an elevation at which these 



animals could not possibly have lived (cf. article " Himalayas," 



Ency. Brit., by Gen. Strachey), show that the vertical uplift of Tibet 



took place in comparatively recent geological times. When this 



tableland had an elevation of a mile or less, it was no doubt inhabited 



^ In § 7 we have cited Humboldt's remarks about volcanoes in parallel 

 ranges being connected by cross ranges, forming mountain-nodes. These 

 are similar to spurs, and hence we see why eruptions occur at such points. 



