580 J. CXIFTOST WAKD ON THE GKANITIC, GKANTTOID, AND 



Pig. 1. — Diagram illustrating the Action of Pressure on Granite in 

 Process of Consolidation. 



subjacent mass and the solid crust were supported like an arch. 

 As soon, however, as upheaval commences, the effective pressure 

 must be at least equal to that which the overlying rock can resist, 

 and the difference between the total calculated pressure and that 

 represented by the thickness of overlying rock must represent its 

 tensile strength. Hence we may further conclude that solidification 

 of the quartz was approximately completed before the total pressure 

 was thus diminished in amount by the fracture of the overlying 

 mass and loss of tensile streugth. If in the above example the total 

 pressure was only 30,000, no upheaval could take place. 



a. Wastdale-Head Granitic Veins. — It will be seen from the 

 table given on p. 575 that the value of v is the same as in the 

 case of the main Eskdale mass. The veins from which this result 

 was gathered are very small strings running among the meta- 

 morphosed volcanic rocks and proceeding from the granitic mass of 

 "Wastdale Head, which is separated from the great granitic area of 

 Eskdale by at least two miles of highly altered rocks, probably 

 underlain at no great depth by granite. 



b. Quartz Felsite (Mvanite) Dykes from the Eskdale Granite. — The 

 dykes yielding the results given in the table (p. 575) are closely 

 associated with the Eskdale Granite. The calculated pressure from 

 the value of v in this case is 53,000, supposing the temperature of 

 consolidation to have been the same as that of the granite, viz. 360° C. 

 (680° P.); this is 11,000 in excess of the granite. It is impossible 

 for these dykes to have consolidated at a greater depth than the 

 granite from which they proceeded ; and therefore we must look to 

 some other cause for this marked difference of result. The subject 

 will be best considered when we deal with other quartz felsites of a 

 similar character. 



3. Shap Granite. — In the case of the two granitic centres already 

 dealt with we have seen that the microscopic investigation helps to 

 bear out the idea formed, from geological evidence in the field, as to 

 their relative depths of origin. Referring back to the table given 

 on p. 575, we should now expect to find that the value of v for the 

 Shap Granite would be considerably higher than it was found to be 

 in the other cases. Instead of this, however, it is lower than in the 

 case of the Eskdale Granite (the latter being -166 and the former 

 only -150), though higher than that of Skiddaw. 



