SORBY STRUCTURE OF CRYSTALS. 495 



graphy of the Sea, 1st edition, plate xi.). If, as is sufficiently pro- 

 bable, the lava at a great depth extends some distance westward of 

 the exhibition of volcanic activity at the surface, there must be a 

 considerable thickness of rock between it and the bottom of the 

 ocean, or else it could not, as it does, resist the pressure of a column 

 of lava at least 20,000 feet high, when an eruption takes place from 

 the Peak. If a few thousand feet is sufficient for that purpose, 

 when the internal forces are relieved by an eruption of lava near the 

 summit of the Peak, there would be a pressure of an actual column of 

 at least 30,000 feet of melted rock on the lava at the base. Pro- 

 bably, however, part of the lava is at a greater depth than a few 

 thousand feet below the general bed of the ocean, and the pressure 

 may be more when not relieved by an eruption, and therefore it ap- 

 pears to me reasonable to suppose it might in some cases be solidified 

 under double that pressure. At all events the best conclusions we 

 can deduce from this modern volcano agree so well with the amounts 

 calculated from the fluid-cavities in granitic rocks, that I cannot but 

 conclude that the pressure under which granites and elvans were 

 consolidated was of the same order of magnitude as the pressure 

 under which the lava of modern volcanos must be solidified at the 

 foci of their activity, as though these rocks were the unerupted lavas 

 of ancient volcanos, variously protruded amongst the superincumbent 

 strata. 



As is well known, the temperature of rocks increases with the 

 depth ; and it becomes an interesting question to determine whether 

 the rate of increase might give the temperature deduced from the 

 fluid-cavities in the quartz of the trachyte of Ponza, at a depth which 

 would correspond with the amount of pressure deduced from a com- 

 parison with those in the quartz of granite. According to M. Cor- 

 dier (Edinburgh New Philosophical Journal, 1828, vol. iv. p. 273), 

 the rate of increase is not uniform in all countries, being in some as 

 rapid as 1° F. for each 24 feet, and in others not more than 1° for 

 each 104 feet, as if owing to an irregular distribution of the subter- 

 ranean heat. If the increase was the same for great depths, there 

 would be a temperature of 680° F. at a depth varying from 15,100 

 to 65,500 feet. According to Mr. R. W. Fox (British Association 

 Report for 1857, p. 96), the rate of increase in various mines in 

 Cornwall is by no means uniform, but varies from 1° for each 32 feet 

 to 1° for 71 feet, being on an average 1° for 49 feet, which would 

 give a temperature of 680° at a depth of 30,900 feet. However, he 

 states expressly that the increase is more rapid in shallow than in 

 deep mines ; and, according to information kindly furnished to me 

 by Mr. Robert Hunt, the rate is 1° for every 50 feet in penetrating 

 through the first 100 fathoms; for the next 100 fathoms 1° for 70 

 feet ; whilst, when the depth exceeds 200 fathoms, it is only 1° for 

 each 85 feet of depth. If this be the true rate of increase far below 

 the surface, there would be a temperature of 680° F. at a depth of 

 about 53,500 feet. These results will be best compared with the 

 pressures under which granites were most probably formed, by means 

 of the following Table ; — 



