496 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Depth in Feet. Pressure in feet of Rock. 



Cordier's results 15,100 to 65,500 



Fox's mean 30,900 



Hunt's results 53,500 



Various granites .... 32,400 to 78,000 

 Cornish granites.... 32,400 „ 63,600 

 Mean of ditto 50,000 



It will thus be seen that, if the rate of increase to a very great 

 depth is the same as near the surface, the calculated temperature 

 would occur at a less depth than corresponds with the calculated 

 pressure, although the general order of magnitude of the two quan- 

 tities is very similar ; whilst, if the rate of increase to a very great 

 depth is the same as below 200 fathoms, it would occur in Cornwall 

 at a depth which corresponds remarkably well with the calculated 

 mean pressure for the granites in that district. Very variable ele- 

 ments enter into the calculations ; there are many possible sources 

 of error ; the number of feet of rock expressing the pressure might 

 differ very considerably from the actual depth, and the distribution 

 of heat in the earth's crust, when the granite was consolidated, 

 might not be the same as now. Still, however, we must admit that 

 the rate of increase in the heat of the rocks indicates that the tem- 

 perature at which the quartz of granite probably crystallized would 

 in all probability occur at a depth agreeing very well with the pres- 

 sure to which it appears to have been exposed. Since, then, as I 

 have already shown, the metamorphic rocks near the granite cry- 

 stallized at about the same temperature as the granite itself, I think, 

 even if we do not give our entire assent, we must acknowledge that 

 the above fact is a strong argument in favour of the supposition 

 that the temperature concerned in the normal metamorphosis of 

 gneissoid rocks was due to their having been at a sufficiently great 

 depth beneath superincumbent strata. 



It will, therefore, be seen that the application of the principles I 

 have described leads to many very striking and remarkable results, 

 which agree so extremely well amongst themselves and with other 

 general circumstances, that I cannot for one moment believe them to 

 be the effect of accident. On the contrary, they clearly point to 

 definite laws ; and though, in the infancy of such a wide subject, in- 

 volving many very difficult physical questions, considerable errors 

 cannot be avoided, yet the character of the results indicates that the 

 general principles are correct. 



With respect, then, to minerals and rocks formed at a high tem- 

 perature, my chief conclusions are as follows. At one end of the 

 chain are erupted lavas, indicating as perfect and complete fusion as 

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

 having 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 intimately associated with 

 the melted rock at great depths was given off as vapour whilst the 

 rock remained fused, the structure is analogous to that of furnace 

 slags. If, however, the pressure was so great that the water could 

 not escape as vapour, it passed as a highly heated liquid holding 

 different materials in solution up the fissures in the superincumbent 

 rocks, and deposited various crystalline substances to form mineral 



