ASSOCIATED METAMOEPHTC EOCKS OP THE LAKE-DI3TEICT. 571 



Silurian strata, it is impossible to connect this granitic mass directly 

 with volcanic phenomena taking place in that period. 



5. Summary. — Thus the possible and probable depths at which 

 these various granitic masses may have been formed are as follows : — ■ 



feet. 



Skiddaw Granite, possibly less than 12000 



more likely 24000 



perhaps most likely 30000 



to 38000 



Eskdale Granite, possibly less than 12000 



most likely 22000 



Shap Granite, most likely 14000 



These figures can of course only be taken as very rough approxi- 

 mations from evidence gathered in the field. It is quite possible 

 that the thickness of the Upper Silurian in its former extension 

 northwards was far less than in the Kendal district ; or the re- 

 verse may have been the case. The estimated thickness of the 

 Yolcanic Series is probably not exaggerated, but it may be under- 

 rated. That of the Skiddaw Slates is difficult to determine with 

 any degree of accuracy, owing to the contorted character of the 

 beds, the absence of definite and traceable divisions, and the want 

 of a base-line to the whole series ; but it seems likely that the 

 thickness of Skiddaw Slates known in the district is not less than 

 10,000 or 12,000 feet*. 



We have now to see how far an examination of the liquid- cavities 

 in the quartz of these granites and other allied rocks throws light 

 upon the question of depth of origin. 



II. Mode of Microscopic Examination, and Precautions taken* 



Mr. Sorby's classical paper has been my main guide in conducting 

 this examination ; but it will be as well to state briefly the process 

 employed and the chief difficulties met with in endeavouring to 

 ensure accurate results. 



All the specimens examined have been in the form of thin slices, 

 which have been viewed by transmitted light. The power used 

 has been a ^-inch (of Collins), which, with a C eyepiece, magnifies 

 665 times. 



^ By employing a Jackson's micrometer with the C eyepiece, the 

 divisions being equal to T o<hnr of an inch, very small cavities 

 and their contained vacuities may be measured. 



A necessary result of the examination of thin slices is, that a very 

 large proportion of cavities may be either wholly or partially drained 

 of their liquid ; hence in the latter case the bubbles would appear 

 larger than they ought to appear. Besides this there may sometimes 

 be cases of air or vapour caught up originally with the fluid. As ex- 



* See Official Memoir on the Geology of the Northern part of the Lake 

 District. 



