128 GEOLOGY OF THE HIGH PLATEAUS. 



the volume of steam ordinarily given off by volcanoes. Unless the esti- 

 mates of observers are altogether deceptive, the quantity of water blown out 

 of volcanic vents must bear a far greater ratio to their lavas than one or two 

 per cent., and we seem to be compelled to assume that the lavas derive their 

 water from extraneous sources, and the penetration of surface water to 

 regions of volcanic energy is by far the easiest explanation. The penetx*a- 

 tion of water, then, is a consideration of importance, but the precise nature 

 of its effects we have no means of determining, and any attempt to follow 

 them would lead us into discussions too purely speculative to be of value. 

 The relief of pressure is another possible mode of liquefying rock. It 

 is postulated by Mr. Clarence King as a basis of his theory of volcanic 

 eruptions. This relief is effected through the removal of superincumbent 

 strata by the process of denudation. Such removals have taken place upon 

 a vast scale, and though geologists have possibly been suspected by other 

 scientists of helping themselves very liberally to a supply of cause and 

 effect of this kind, yet the surveys of our western domain have proven that 

 they have been very modest and abstemious. But that such a process 

 could have played a very important, much less a fundamental, part in caus- 

 ing volcanic eruptions seems to be negatived by facts. We do not find that 

 eruptions always occur in localities which have suffered great denudation. 

 We do not find even that they occur in such localities predominantly. Most 

 of the existing volcanoes and most of those which have recently become 

 extinct are situated in regions which have suffered very little denudation in 

 recent geological periods, and many of them in regions of recent deposi- 

 tion. JEtna is built upon a platform of Post-Tertiary beds and Vesuvius 

 stands upon late formations. The same is true, according to Dr. Junghuhn, 

 of the volcanoes of Java, and this fact is repeated in the great volcanoes of 

 the Cape de Verde and Canary Islands. The High Plateaus of Utah, 

 which have been the theater of volcanic activity since the Middle Eocene, 

 are localities of minimum erosion, while the denudation of the non- volcanic 

 regions around them has been stupendous. It can hardly be supposed that 

 the volcanoes of the Pacific have broken forth from denuded localities, 

 unless the denudation took place at a considerable period of past time. 



But whatever may be the effects of the relief of pressure, and how- 



