494 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



a conclusion wliicli appears to me to agree extremely well with the 

 general association of facts on a large scale. 



Of course all the pressures deduced as above involve any error 

 there may be in the amount of compression supposed to be the most 

 probable for highly heated water, and would be materially altered 

 by variations in the temperature ; but, considering all the circum- 

 stances of the case, it appears to me as likely that they are too little 

 as too great, and, therefore, at present we cannot do better than 

 adopt them provisionally. 



In order that the various results may be compared more conve- 

 niently, I subjoin the following Table. The first column gives the 

 temperature in degrees Centigrade requisite to expand the fluid so 

 as to fill the cavities, if the pressure was not greater than the elastic 

 force of the vapour, which, of course, is the lowest temperature at 

 which the rock can have been consolidated, since the excess of pres- 

 sure could not be less than nothing. In the other column is given 

 the pressure in feet of rock requisite to compress the fluid so much 

 that it would just fill the cavities at 360°, being, therefore, the actual 

 pressure, if in each case the rock was consolidated at that temperature. 



Tempera- 

 ture. Pressure. 



Trachyte of Ponza 356 4,000 



Elvan at Gwennap 320 18,100 



Granite at St. Austel 256 32,400 



Mean of the Cornish elvans 250 40,300 



More recent veins of granite at Aberdeen 245 42,000 



Mean of Cornish granites 216 50,000 



Elvan at Swanpool, near Falmouth 203 53,900 



Granite from the Ding Dong Mine, near Penzance .... 162 63,600 



Mean of the Highland porphyry-dykes 135 69,000 



Exterior of the main mass of the granite at Aberdeen 135 69,000 



Mean of the Highland granites 99 76,000 



Centre of the main mass of the granite at Aberdeen ... 89 78,000 



It will thus easily be seen that, if pressure is not taken into 

 account, there is a gradual decrease in temperature on passing from 

 trachyte to granite ; whilst if, as is far more probable, the temperature 

 was nearly the same, the pressure increases in passing from trachyte 

 through elvans to granite; and I think all geologists will agree with 

 me in thinking that this is a very satisfactory result. 



It therefore appears that the fluid-cavities indicate that all the 

 elvans and granites I have hitherto examined were consolidated 

 under pressures varying from about 18,000 to 78,000 feet of rock. 

 These are certainly very great pressures j but, bearing in mind that 

 they probably represent the forces concerned in the elevation of 

 mountains, I think they are sufficiently reasonable. They also cor- 

 respond very well with the pressure under which, in many cases, the 

 lava at the feci of modern volcanic activity must become solid, as 

 is well illustrated by the Peak of Teneriffe. It is upwards of 12,000 

 feet high, and the bottom of the ocean from which the volcanic dis- 

 trict of the Canary Islands rises is 12,000 feet deep, and at no great 

 distance westward it is 10,800 feet (Lieut. Maury's Physical Geo- 



