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



In the valley of the Ganges the earthquake thrusts were towards 

 the Himalayas, and at some period in the upheaval of these moun- 

 tains the crust may have been cracked and also so pulled apart as 

 to allow the great lava flows observed in the plateau of Deccan. 

 Much of this may have occurred when the peninsula of India was 

 still under the sea. No doubt the great lava flows of Oregon and 

 Utah were similar in character, but the details of the process must 

 be left to future investigation. 



§ 47. On the vapors exhaled by -volcanoes. 



We have seen that, according to Geikie, 999 parts in 1,000 of the 

 vapors emitted by volcanoes is composed of steam, and have con- 

 cluded that this is the only original vapor operating under the earth's 

 crust. For the one-thousandth part of other gases such as sulphur- 

 ous oxide, carbon dioxide, hydrochloric acid, et cetera, may well 

 be derived from the heated rocks when saturated with steam, with- 

 out any other original gas existing in the earth. If any other gas 

 except steam were really active in the earth, either under the vol- 

 canoes or in remote regions which are disturbed by earthquakes, we 

 may be very sure that at some of the volcanoes some of this gas 

 would escape and we should be able to recognize it. As such escape 

 does not occur we may be quite sure that steam is the only original 

 vapor operating within the earth. 



The question of the dissociation of water vapor by the intense 

 heat of subterranean lava is also worthy of remark. It has been 

 found by analyses of the vapors escaping from Thera, where the 

 outlets are generally submarine, that the water gases are dissociated. 

 Immediately over the focus of eruption free hydrogen formed 30 

 per cent, of the gases emitted. In general the free hydrogen is fully 

 twice as abundant as the free oxygen, so that the mixture on coming 

 in contact with a burning body at once ignites with a sharp ex- 

 plosion. A considerable quantity of free nitrogen is also present, 

 and traces of other gases. Fouque, who has given most attention 

 to this subject, infers that the water- vapor of volcanic vents may 

 exist in a state of dissociation in the magma just beneath the earth's 

 crust. The free nitrogen is supposed to be derived from the air 

 absorbed in the water which percolates downward. 



The question of the existence of true volcanic flames was first 



