1864.] T. STERRY HUNT ON LITHOLOGY. 19 



tic force of the vapor. This of course represents the lowest temper- 

 ature at which the consolidation could have taken place, and 

 varies from 340° to 380° in the Vesuvian minerals, and 356° 

 in the quartz of the trachyte of Ponza, to a mean of 216° in the 

 Cornish granites, to 99° in those of the Scottish Highlands, and 

 even descends to 89° in some parts of the granite of Aberdeen. 

 But this low temperature is improbable, and inasmuch as water 

 and aqueous solutions are compressible, their volume would be 

 considerably reduced under a great pressure of superincumbent 

 rock. Mr. Sorby has therefore calculated the pressure in feet of 

 rock which would be required to compress the liquid so much that 

 It would just fill the cavities at 360° C. The numbers thus ob- 

 tained will therefore represent the actual pressure, provided the 

 rock was in each case consolidated at that temperature. It would 

 thus appear that the trachyte of Ponza was solidified near the sur- 

 face, or beneath a pressure of only 4000 feet of rock ; while for the 

 Aberdeen granite the pressure was equal to not less than 78,000 

 feet, and for the mean of the Highland granites Y6,000. The 

 Cornish granites vary from 32,400 to 63,600, and give as a mean 

 50,000 feet of pressure. In this connection Mr. Sorby remarks 

 that from Mr. Robert Hunt's observations on the mean increase of 

 temperature in the mines of Cornwall, a heat of 360° C. would 

 be attained at a depth of 53,500 feet. 



The observations upon the metamorphic crystalline schists in 

 the vicinity of these various granites show that their constituent 

 minerals must have crystallized at about the same tempera- 

 ture as the granite itself; affording, as Mr. Sorby observes,-" a 

 strong argument in favor of the supposition that the temperature 

 concerned in the normal metamorphism of gneissoid rocks was due 

 to their having been at a sufiiciently great depth beneath superin- 

 cumbent strata"; and he concludes that with regard to rocks and 

 minerals formed at high temperatures, we have " at one end of the 

 chain erupted lavas, indicating as perfect and complete fusion as 

 the slags of furnaces, and at the other end simple quartz-veins, hav- 

 ing a structure precisely analogous to that of crystals deposited 

 from water. Between these there is every connecting link, and 

 the central link is granite." When the water, which at great 

 depths was associated with the melted rock, was given off as vapor 

 while the mass remained fused, slag-like lavas resulted. If 

 however the water could not escape in vapor, it remained, as we 



