ORIGIN OF VOLCANIC FISSURES. 175 



and multitudes of hair-like crystals of schorl are enclosed in the 

 quartz of Cornwall, it is inferred that the granite did not become 

 finally solid at a temperature much higher than a dull red heat. 

 The temperature inferred for an elvan dyke from the fluid cavities 1 

 is 608 F., which indicates a pressure of 18,100 feet; but most of 

 the observations on Cornish elvans gave Dr. Sorby a pressure of 

 40,300 feet, while the quartz-porphyry dykes of the Highlands 

 indicate, on similar evidence, a pressure of 69,000 feet. The granite 

 of St. Austell in the same way indicates a temperature of 490 F., and 

 a pressure of 32,400 feet, while near Penzance the pressure cor- 

 responds to 63,600 feet; the mean pressure indicated by Cornish 

 granites is 50,000 feet. The mean pressure of the Aberdeen granite 

 is about 76,000 feet, while the centre of the main mass of the granite 

 of Aberdeen requires a pressure of 78,000 feet. Whence we learn 

 that the inferred temperatures, under which these rocks were pro- 

 duced, are scarcely higher than would be reached at corresponding 

 depths beneath the surface by the mere natural augmentation of the 

 earth's heat, so that if anything like 50,000 or 70,000 feet of rock 

 has been denuded to expose the granite, all difficulty as to the 

 temperature vanishes ; and the water, though greatly heated, was 

 in most cases caught up by the crystals in a fluid state, more or less 

 saturated with the alkalies which enter into the composition of the 

 minerals forming the rock. 



How Upheaval Facilitates the Outburst of a Volcano. When a 

 great upward fold of the earth's crust is in process of formation or further 

 elevation beneath the sea, two things inevitably happen : first, the 

 external surface of the rocks is stretched, and therefore fissured; 

 and the greater the elevation, the deeper and more numerous these 

 fissures must become. Down such cracks water would inevitably 

 penetrate, and modify the condition of heated rocks with which it 

 came in contact. Its presence in this form was probably unnecessary, 

 as we may hereafter show, to the production of any crystalline rock ; 

 but such infiltrated water initiated changes which modified a rock, 

 that might have become crystalline, into a fluid and eruptive form. 

 And secondly, as the upheaved part of the earth's crust approached to- 

 wards the surface of the ocean, it became cut down by denudation 

 into a plain, so that an immense thickness of sediments was removed 

 from above the central axis, where heat was already developed by 

 a lateral compression ; and when relieved of this superincumbent 

 pressure, and the weight of the water by emerging from the sea, the 

 pressure above the heated mass is so far reduced, that the expansive 

 force of the steam and liquid rock overcomes resistance, and a volcanic 

 eruption is possible. No small number of volcanoes exists in table- 

 lands, or among mountains near to the sea. And if we find that 

 granite, for instance, has become liquid and is poured out on the surface 

 of the earth as a volcanic rock, and that the parent masses from which 



1 For a discussion of nature of this evidence, see Sorby on the Microscop'cal 

 Structure of Crystals, &c., Q. J. G. S , vol. xiv. p. 453. 



