ASSOCIATED METAMOEPHIC EOCKS OP THE LAKE-DISTRICT. 569 



3. Shap Granite. 



4. Ennerdale and Buttermere Syenite and Syenitic Granite, 



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



a. Quartz Felsite Dyke of Armboth and Helvellyn. 



6. Quartz Felsite of Fairy Crag. 

 General Remarks. 



IV. Summary. 



Introduction. 



ISTo one can read Mr. Sorby's masterly and elaborate paper on the 

 Microscopical Structure of Crystals * without being struck with the 

 amount of patient investigation it displays and the host of interesting- 

 questions it suggests. During the last few years that it has been 

 my privilege to work in the field among the rocks of the Lake- 

 district it has frequently been upon my mind to apply Mr. Sorby's 

 method of investigation to the granites and other quartz-bearing 

 rocks, in order to see how far inferences drawn from field-observations 

 were confirmed by the microscopic study of the liquid-cavities and 

 their contained vacuities. I have been the more anxious to do this 

 as the relations which the three granitic centres of the district bear 

 to the surrounding rocks vary in each case, and I think there is 

 good reason to infer from geological evidence that the three masses 

 solidified under different pressures and conditions. 



I. Geological Relations of the three Granitic Centres. 



1. General Relations. — The Skiddaw Granite occurs only in con- 

 nexion with the Skiddaw Slates, which are extensively metamor- 

 phosed around it. 



The Eskdale Granite is wrapped round by bighly altered rocks of 

 the Volcanic Series of the Lake-district. 



The Shap Granite is also for the most part surrounded by volcanic 

 rocks metamorphosed in a similar manner to, but less widely than, 

 those around the much larger Eskdale mass ; on the south, however, 

 the granite is in close connexion with the lowermost beds of the 

 Coniston Series, which are altered by it ; and granitic dykes occur 

 amongst the Upper Silurian strata. 



Various questions are suggested by these relations : — At what 

 period was each granitic mass formed? Was the thickness of 

 superincumbent rocks at all similar in the three cases ? "Were any 

 or all of these granitic masses connected with the formation of the 

 volcanic rOcks ? 



2. Skiddaw Granite. — The Skiddaw Granite, occurring as it does 

 in several inlying masses of small extent (the largest but one mile 

 in length and half a mile in breadth), and being surrounded on all 

 sides by great thicknesses of metamorphosed Skiddaw Slates, would 

 seem to have been solidified in connexion with the Skiddaw Series 

 alone ; and, had denudation been carried a little less far than it has 

 been, the existence of granite immediately below would only have 



* Quart. Journ. Geol. Soc. vol. xiy. p. 453. 



