586 



J. CLIFTON WARD ON THE GRANITIC, GRANITOID, AND 



v p 



Eskdale Granite -166 42,000 



„ „ veins -166 42,000 



„ „ , quartz-felsite dykes from . . -125 53,000 



Quartz Pelsite of St. John's -170 40,000 



„ „ dyke of Armboth -150 46,000 



This gives an excess of 11,000 in the one case and of 6000 in the 

 other. Mr. Sorby has shown that while the granite of St. Austel, 

 in Cornwall, indicates a pressure considerably less than some of the 

 elvans, yet the mean of all the observations on the Cornish gra- 

 nite is 9700 feet more than the mean for the elvans ; and upon this 

 he remarks: — "This, I think, is a very satisfactory result, since 

 the association of these rocks clearly proves that granite must have 

 been consolidated at a considerably greater depth than elvans." 



The conditions of consolidation in such narrow dykes as most of 

 these are must certainly be very different from those in a large mass ; 

 and in most cases it would seem probable that the rate of cooling 

 must have been very rapid, taking place between vertical walls of 

 rock at a vastly lower temperature, and that the pressure on all sides 

 must have been very great and perhaps scarcely comparable with the 

 pressure acting upon a mass of solidifyiug granite. 



6. Quartz Fehite of Fairy Crag. — This is a small area of rock, 

 very similar, on the whole, to that of St. John's Vale, and occurring 

 solely among the Skiddaw Slates at some little distance from the 

 edge of the Volcanic Series. It will be seen from the table on p. 575 

 that the value of v rather nearly approaches that of the Skiddaw 

 Granite, the former being '140 and the latter *130. This relation 

 would indicate, if the microscopic results be at all trustworthy, that 

 the two rocks were consolidated under somewhat similar pressures; and 

 accordingly we find the following when tabulated and compared : — 



9. Skiddaw Granite. 

 •130. If p — the min. temp. 

 =208° C. (406° F.). 



g^ 



%1 



o 



Top of Upper Silurian. 



.30,000. 



Fox's calculated 

 temperature at 

 this depth, 350°C. 

 (662° F.). 



*;=-130; 

 .*. calculated pressure (p) =52,000. 

 Tbis gives : — 



Sorby's estimated 

 temperature of 

 solidification of 

 granites, 360° C. 



10. Quartz Felsite of Fairy Crag. 



v=-140. If p=0, the min. temp. 

 =218° C. (424° F.). 



^ : ^ 



Top of Upper Silurian. 





.27,000. 



Fox's calculated 

 temperature at 

 thisdepth,315°C. 

 (600 F.) 



Sorby's estimated 

 temperature of 

 solidification of 

 granites, 360° C. 

 (680° F.). 



.-. calculated pressure (p) =49,000. 

 This gives :— 



