88 



by Mr. Sorby's investigations, the temperature of a dull red heat 

 visible in the dark (68o°F.) being assumed as the probable 

 temperature of consolidation. The result, in the case of the 

 Skiddaw Granite, is that a pressure equivalent to 52,000ft. of rock 

 must have been necessary to compress the liquid so that it would 

 fill the cavities at a temperature of dull red heat. 



The next question, bearing upon this result, is one for the 

 field-geologist alone to determine. What is the greatest thickness 

 of rock which can have been removed from over the mass of the 

 Skiddaw granite as we now see it exposed ? Geological investi- 

 gation of the whole district leads me to infer that at one time the 

 granite must have been covered by some 30,000 feet of rock — 

 including Skiddaw Slates, "Volcanic Series, and Upper Silurians. 

 But we have seen that the calculated pressure is equivalent to 

 52,000 ft, hence the pressure to which the consohdating granite 

 (and therefore quartz) was subject was greater than what could be 

 due to the mere weight of overlying rocks. How then was this 

 pressure applied'? We have seen that the condensed liquid 

 confined within the quartz has remained as a registering ther- 

 mometer, to show the existence and amount of the pressure, 

 and geological examination of the District further informs us that 

 the overlying 30,000 feet of rock was in some way elevated, and 

 contorted — being also slowly removed by denudation. Thus we 

 have physical evidence from two different sources of the existence 

 of great pressure exerted upon the granite-forming mass ; evidence 

 derived from the liquid-cavities and their contained bubbles, and 

 evidence in the rocks overlying the granite of such pressure having 

 produced elevation, contortion, cleavage, and general meta- 

 morphism. It is further interesting to notice what would be the 

 probable heat at a depth of 30,000 feet, according to our estimates 

 of known increase on descending through the crust of our globe. 

 If we take that increase as 1° Fah. for every 49ft. (Mr. R. W. Fox, 

 Brit. Assoc. Report, 1857, p. 91), we find that at a depth of 30,900ft. 

 the temperature would be 360° C. (680° F.), or that of a dull red 

 heat visible in the dark, and just the temperature at which we 

 were supposing the granite was formed. Thus, to sum up our 

 results in the case of the Skiddaw Granite : — 



