492 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



rably accurate approximation to the truth when the pressure was 

 very great, as well as when only small. Even if the real amount of the 

 compression of the highly heated liquid differed from that supposed 

 to be the most probable, the difference would affect all the results in 

 similar proportion, and therefore, though not actually correct, they 

 would be sufficiently accurate as compared with each other. The 

 chief point about which there may be some doubt is whether, when 

 the rock contains much quartz, it became crystalline at a higher 

 temperature than when it contains less. I have, indeed, found cases 

 where there was evidence of the first-formed quartz having crystal- 

 lized at a higher temperature than the last, but the facts were scarcely 

 sufficiently decided to be fully relied on, and in the present state of 

 the inquiry this cannot be accurately taken into account ; nor, indeed, 

 if mean results are employed, do any other facts seem to require that 

 it should. That even highly quartzose el vans and granites did not 

 become finally solid at a temperature much higher than a dull-red 

 heat, is, I think, clearly proved by the great number of hair-like 

 crystals of schorl enclosed in the quartz ; for schorl readily melts at 

 a bright-red heat, and therefore must have crystallized at a lower 

 temperature than that. The properties of the pyrognomic minerals 

 described by Scheerer (Discussion sur la nature plutonique du granite, 

 &c, ut supra) indicate a temperature not higher than a brown-red 

 heat. It therefore appears to me in the highest degree probable 

 that granites and elvans became finally solid at about the dull-red 

 heat calculated from the fluid-cavities in the quartz of the trachyte 

 of Ponza. Still, however, taking everything into consideration, the 

 following deductions must only be looked upon as the best approxi- 

 mations that can be made at present, for so many data are only im- 

 perfectly known. 



From the nature of the case, equation (10) gives the excess of 

 pressure over and above that under which the quartz of the trachyte 

 of Ponza crystallized, whatever, within moderate limits, the real 

 temperature and pressure might be. If then we consider these to 

 have been 360° C, and 4000 feet of rock, that amount would have 

 to be added to the calculated value of p, in order to obtain the total 

 pressure. 



The greatest value of v that I have yet found for any elvan is for 

 one at Gwennap, in which it is very nearly '25. This, from equa- 

 tion (8), indicates a minimum temperature of about 320° C. (608° F.), 

 or very little lower than a dull-red heat visible in the dark. Sub- 

 stituting this value of v in equation (10) we obtain p= 14,100 feet, 

 to which, as explained above, must be added 4000 feet to arrive at 

 the total pressure, which was therefore about 18,100 feet. The 

 least value of v for any elvan in Cornwall is "125, for that at Swan- 

 pool, near Falmouth, which corresponds to a pressure of 53,900 feet. 

 The mean of my observations in the elvans of Cornwall gives a pres- 

 sure of 40,300 feet ; but, for the analogous quartzose porphvry-dykes 

 in the Highlands of Scotland, 69,000 feet. 



I have never yet found any granite in which v is greater than '2, 

 which is the relative size of the vacuities in that of St. Austel. This 



