THE LAKE DISTRICT GRANITES. 235 



Granitic Rocks of the Lake District. 



The igneous rocks of the Lake District were well described by the 

 late Mr. Clifton Ward. 1 



The Skiddaw Granite occurs in small inlying masses, the largest 

 being only a mile long and half a mile wide. It is seen in the valley 

 of the Caldew, and in the course of Sinen Gill, and is everywhere 

 surrounded by Skiddaw slates, which are greatly metamorphosed by the 

 contact. There is no evidence that these granite masses were the cores 

 of old volcanoes, since the volcanic rocks which occur in the surround- 

 ing strata are more basic and chiefly belong to the basaltic family. 



The felspar is partly orthoclase and partly triclinic, the latter 

 showing the usual coloured bands when seen under polarised light in 

 microscopic sections. 



The quartz contains many fluid cavities with moving bubbles 

 and hair-like crystals, as well as others which are stout and long. 

 The mica is mostly dark-brown. There are a few grains of magnetite. 

 Mr. Ward infers that the Skiddaw granite has never been covered by 

 a greater thickness of strata than 38,000 feet. 



The liquid cavities in the quartz are considered to indicate a 

 pressure equal to 52,000 feet of superincumbent rock, it being supposed 

 that this pressure was exerted in the upheaval of the overlying rocks, 

 and that the Skiddaw granite consolidated at a temperature of about 

 680 Fah. 



The Eskdale Granite is only met with surrounded by the volcanic 

 series of Borrowdale. From the thickness of these strata it is con- 

 cluded that not more than 22,000 feet of rock could ever have covered 

 the granite. The granite metamorphoses the rocks with which it is 

 in contact. The pressure which is indicated by the condition of the 

 crystals is stated by Mr. Ward at 42,000 feet. Dykes of quartz-felsite 

 or elvanite from this rock indicate a pressure of 53,000 feet. 



The Wastdale Granite is another mass, coloured reddish by its 

 felspar, which is partly orthoclase and partly triclinic, and contains 

 dark-brown mica. An analysis shows it to contain, as compared with 

 the Skiddaw granite, rather less silica and more alumina, less potash 

 and more soda and more iron. 



The Wastdale granite is assumed to be in connection with the 

 Eskdale granite. At Wastdale Head numerous veins run into the 

 overlying volcanic series. 



The Snap Granite, so well known on Wastdale Crag and Wastdale 

 Pike, is characterised by large flesh-coloured or reddish-brown crystals 

 of felspar. Like the other masses, it is incapable of being connected 

 with any volcanic outburst. The geological evidence indicates that 

 it could not have consolidated under a greater pressure than about 

 14,000 feet of superincumbent strata, but the calculated pressure 

 indicated by the microscopic structure is 46,000 feet, at a temperature 

 of 502 Fah. This granite is inferred to have cooled very slowly. 

 The metamorphism in the surrounding rocks is less intense than in 



1 Quarterly Journal Geological Society, vol. xxi. p. 568 ; vol. xxxii. p. i ; Me- 

 moirs of the Geological Survey : Northern Part of the English Lake District, 1876. 



