ASSOCIATED META110RPHIC ROCKS OF THE LAKE-DISTRICT. 



579 



«/=-130. 



.-. Calculated pressure (p) =52,000. 

 This gives : — 

 Downward pressure Surplus pressure 



| | 



30,000. 22,000. 



.-. Calculated pressure (p) =42,000. 

 This gives: — 

 Downward pressure Surplus pressing 



I I 



22,000. 20,000. 



Prom the nature of the investigation, to quote Mr. Sorby's words, 

 " the true heat can only be determined when the approximate value 

 of the pressure is known ; and the pressure cannot be deduced unless 

 we can in some way or other approximate to the temperature." But 

 the pressure, since it does not necessarily mean the mere thickness 

 of overlying rocks, contains two elements, only one of which can be 

 arrived at by purely geological evidence ; so that, arriving at the other 

 element by microscopic investigation, it should, if reliance can be 

 placed on the microscopic results, when taken together with the 

 temperature, be able to effect an equivalent amount of work in the 

 various cases dealt with. For example, if an upward, or surplus, 

 pressure of 22,000 feet at 360° C. elevate 30,000 feet, what should 

 be the temperature for an amount of upward pressure represented 

 by 20,000 to elevate 22,000 feet? A simple calculation gives 

 290° C. as the necessary temperature. Or, if 30,000 feet are elevated 

 at a temperature of 360° C. by an upward pressure of 22,000 feet, 

 what amount of upward pressure is required to elevate 22,000 feet 

 at the same temperature ? The answer is 16,000. This last result, 

 put in a tabular form, appears thus : — ■ 



3. Skiddaw. 

 Downward pressure (D. P.) =30,000. 



I 



£=360° C. 

 ^=•130 

 .\p= 52,000. 



Surplus pressure (S. P.) =22,000. 



4. EsJcdale. 

 D. P. =22,000 



I 



tf=360° C. 

 .-.#=42,000. 



1 



S. P. =16,000. 



The discrepancy between the value of 22,000 + 16,000 and 42,000 

 is not very great when the uncertainty of some of the data is 

 remembered. 



It is possible that the use of the terms downward pressure and 

 surplus or outward pressure, and that of the arrows as symbols, 

 while helping to give clearness in one direction may impart ambi- 

 guity in another. The case may be put in another form as follows : — 



Let the arch a b (fig. 1, p. 580) represent a thickness of 30,000 feet 

 of rock overlying the consolidating granite z, and the lines x, y re- 

 present pressure from below. If the liquid- cavities show a total pres- 

 sure in feet of rock of 50,000, that pressure must be dependent on 

 the tension of the overlying rock, and might be reduced to less than 

 what corresponds to 30,000 feet if there were contraction of the 



