578 



J. CLIFTON WAKD ON THE GItANITIC, GKAN1TOID, AND 



2. Eskdale Granite. — It has already been shown that, in all 

 probability, judging from purely geological evidence, the Eskdale 

 Granite was consolidated at a less depth than that of Skiddaw, 22,000 

 feet being a probable estimate. We should therefore expect to find 

 that the microscopic evidence would likewise show a less pressure ; 

 and this is found to be the case, thus : — 



Depth. Pressure. 



Skiddaw Granite 30000 52000 



Eskdale Granite 22000 42000 



The same argument may be applied here as in the case of the 

 Skiddaw Granite. The calculated pressure being so much in excess 

 of the estimated thickness of overlying strata, it is unlikely that the 

 pressure was relieved by volcanic action ; and it is rendered more 

 probable that the excess of pressure was spent in the work of eleva- 

 tion and contortion. Now it is curious to observe that the excess in 

 this case is nearly the same as in the last, viz. 20,000 feet as against 

 22,000 ; but whereas in the former a pressure equal to 22,000 feet 

 in rock at 360° C. (680° E.,) elevated a thickness of 30,000 feet of 

 strata, in the latter case one of 20,000 upheaved but 22,000 feet. 

 Are we, however to regard the temperature as the same in both 

 cases ? Taking Fox's mean, the temperature at a depth of 22,000 

 feet would be 259° C. (499° E.). Taking the microscopic value of 

 ^=•166, we obtain from Mr. Sorby's formula (<T), 



t= V684462V+12144-110, 



the result that the temperature requisite to expand the fluid so as to 

 fill the cavity under the pressure of the elastic force of its vapour 

 only is 244° C. (471° E.) ; and therefore under the pressure of 

 42,000 feet the temperature required must have been much greater 

 than 244° C. (471° E.), and consequently very much greater than 

 the calculated temperature (according to Eox) at the depth of 

 22,000 feet. Hence we have this result on comparing the cases of 

 the Skiddaw and Eskdale Granites : — 



1. Skiddaw. 



v = ' 1 30. If p = 0, the minimum temp. 

 =208° O. (406° F.). 



Hi 



; 



Sorby's estimated 

 temperature of 

 solidification of 

 granites, 360 C. 

 (680° F.), 



.30,000. 



Fox's calculated 

 temperature at 

 this depth, 350° C. 

 (662° F.), 



2. Eskdale. 



•166. Ifjp=0, the minimum temp. 



=244° C. (471 °F.). 



6 

 o 



§1 



.8 6 



Hi 



.22,000. 



; 



Sorby's estimated 

 temperature of 

 solidification of 

 granites, 360° 0. 

 (680° F.). 



Fox's calculated 

 temperature at 

 this depth, 259° C. 

 (499° F.). 



