ASSOCIATED METAMOKPHIO ROCKS OF THE LAKE-DISTRICT. 585 



8. Quartz Felsite of St. John's Vale. 

 V—-VIO. If p=0, the min. temp, is 248° C (478° F.). 



Top of Lower Silurian. 



a^ 



& a 



.26,000. 



Fox's calculated temperature at 

 this depth, 304° C. (580° F.).; 



M o 

 O 



Sorby's estimated temperature of 

 solidification for granites j 360° O. 

 (680° F.). 



^ = ■170; 



.•. calculated pressure (p) =40,000. 

 This gives : — ■ 



Downward pressure Surplus pressure 



I I 



26,000. 14,000. 



With regard to this and the other quartz-felsites we have yet to 

 notice, it may be remarked that the crystals of quartz are generally 

 double pyramids without intermediate prismatic surface, and that 

 they have impressed their form upon the felsitic matrix. ISTow a 

 compound of felspar and quartz melts almost as readily as felspar 

 (Cotta) ; and when the silica began to crystallize out, the disen- 

 gaged heat would help to keep the more felsitic base for a time 

 in a partially melted state ; and then solidification would probably 

 take place rather rapidly, owing to the preponderance of felspar. 

 Thus the different appearance of quartz-felsites from granites is very 

 probably due to the final solidification taking place more rapidly in 

 the former than in the latter, owing to the felspar in the one and 

 the quartz in the other being the last to solidify, the felspar not 

 passing through a viscous state. And if we compare two such rocks 

 as the Shap Granite and this Quartz Felsite of St. John's Yale, we 

 may perhaps conclude that the former solidified more slowly and at 

 a higher temperature than the latter, but probably also under a less 

 pressure of overlying rocks. 



a. Quartz . Felsite Dyke of Armboth and Helvellyn. — This very 

 beautiful rock forms a narrow dyke extending for many miles through 

 the Yolcanic Series ; and although it cannot be traced up to the 

 St. John's Quartz Felsite, there is little doubt, judging from its 

 similar character, that it has proceeded from some part of that 

 mass. 



The value of v (-150) given in the table, is the mean of observa- 

 tions taken from widely different parts of the dyke, and points to a 

 pressure of 46,000 feet, or 6000 feet more than the main mass from 

 which it probably proceeded. It will be remembered that somewhat 

 similar dykes running off from the Eskdale Granite showed an even 

 greater excess of pressure over the main mass. Thus we have : — 



