GEOLOGY. 275 



bed of concrete, made of lime, fragments of brick, and sandstone. 

 Through this and other masonry, the hot waters have been percolat- 

 ing for centuries, and have given rise to various zeolites, apophyllite 

 and chabazite, among others ; also to calcareous spar, arragomte, and 

 tlnor spar, together with siliceous minerals, such as opal, all found 

 in the interspaces of the bricks and mortar, or constituting .part of 

 their rearranged materials. The quantity of heat brought into action 

 in this instance, in the course of 2,000 years, has, no doubt, been 

 enormous, although the intensity of it developed at any one moment 

 has been always inconsiderable. 



The study, of late years, of the constituent parts of granite has, in 

 like manner, led to the conclusion that their consolidation has taken 

 place at temperatures far below those formerly supposed to be indis- 

 pensable. Gustav Rose has pointed out that the quartz of granite has 

 the specific gravity of 2'6, which characterizes silica when it is precip- 

 itated from a liquid solvent, and not that inferior density, namely, 2'3, 

 which belongs to it when it cools, and solidifies in the dry way from a 

 state of fusion. 



But some geologists, when made aware of the intervention, on a 

 large scale, of water, in the formation of the component minerals of 

 the granitic and volcanic rocks, appear, of late years, to have been too 

 much disposed to dispense with intense heat when accounting for the 

 formation of the crystalline and unstratificd rocks. As water, in a 



tf 



state of solid combination, enters largely into the aluminous and some 

 other minerals, and therefore plays no small part in the composition 

 of the earth's crust, it follows that, when rocks are melted, water must 

 be present, independently of the supplies of rain-water and sea-water 

 which find their way into the regions of subterranean heat. But the 

 existence of water under great pressure affords no argument against 

 our attributing an excessively high temperature to the mass with which 

 it is mixed up. Still less does the point to which the melted matter 

 must be cooled down before it consolidates or crystallizes into lava or 

 granite, afford any test of the degree of heat which the same matter 

 must have acquired when it was melted and made to form lakes and 

 seas in the interior of the earth's crust. 



We learn from Bunsen's experiments on the Great Geyser in Ice- 

 land, that at the depth of only 74 feet, at the bottom of the tube, a 

 column of water may be in a state of rest, and yet possess a heat of 

 120 Centigrade, or 248 F. What, then, may not the temperature 

 of such water be at the depth of a few thousand feet ? It might soon 

 attain a white heat under pressure ; and as to lava, they who have be- 

 held it issue, as I did in 1858, from the southwestern flanks of Vesu- 

 vius, with a surface white and glowing like that of the sun, and who 

 have felt the scorching heat which it radiates, will form a high concep- 

 tion of the intense temperature of the same lava at the bottom of a 

 vertical column several miles high, and communicating with a great 

 reservoir of fused matter, which, if it were to begin at once to cool 

 down, and were never to receive future accessions of heat, might 

 require a whole geological period before it solidified. Of such slow 

 refrigeration hot springs may be among the most effective instruments, 

 abstracting slowly from the subterranean molten mass that heat which 

 clouds of vapor are seen to carry off in a latent form from a volcanic 



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