236 RHYOLITES IN RELATION TO THE GRANITES. 



the Eskdale district. Patches of fine-grained micaceous granite some- 

 times occur in the middle of the Shap mass. 



The Ennerdale and Buttermere Syenite and Syenitic Granite. 

 The Ennerdale rock is for the most part red and of coarse grain. 

 Usually hornblende is absent or in small quantity, but it is sometimes 

 so abundant as to give with the mica a dark and grey tinge to the 

 rock ; and then its texture is fine-grained. It is well seen over the 

 summit of Red Pike. The quartz appears to occupy the interstices 

 between the other minerals, and is not crystallised. Magnetite is 

 sometimes plentiful. Where the syenite meets the overlying volcanic 

 ash it becomes a hornblendic felsite. It has a distinct aspect from 

 the Eskdale granite, though it adjoins that rock at the foot of Wast- 

 water. It extends north and south for about nine miles, and forms in 

 part a boundary between the Skiddaw slates and the volcanic series. 

 The liquid cavities in the quartz indicate a pressure at consolidation 

 equal to 35,000 feet of rock. This mass may have furnished the 

 ashes in the uppermost part of the volcanic series. 



St. John's Quartz Felsite. This rhyolitic rock varies in colour 

 from red to white, and consists of a felspathic base containing crystals 

 of felspar and quartz, with a little magnesia, mica, and some altered 

 hornblende. The felspar is chiefly orthoclase. Grains of magnetite 

 occur. The quartz contains fluid cavities which indicate a pressure 

 equal to 40,000 feet of rock. The masses in which it occurs are 

 rarely more than a mile long. The crystals of quartz in this rock are 

 generally double pyramids, the intermediate prism not being developed. 

 This formation of complete crystals is difficult to reconcile with the 

 theoretical estimate of pressure. 



The Armboth and Helvellyn Dyke. This rock also consists of 

 quartz felsite or rhyolite, and is traced towards the St. John's rock. 

 The dyke, which is 20 to 30 feet wide, is a red felspathic base, with 

 crystals of pink felspar and quartz, with green mica and a steatitic 

 mineral. Most of the quartz is crystallised, but some is interstitial. 

 The percentage of silica is less than in any of the other rocks. The 

 pressure indicated by this rock corresponds to 46,000 feet, it being in 

 accordance with other observations that the pressure is greater in a 

 dyke than in the mass from which it proceeds. 



Quartz Felsite of Fairy Crag. This is a similar area to that 

 of St. John's Vale, and occurs among the Skiddaw slates. It in- 

 dicates pressure of 49,000 feet. These rhyolitic rocks may be re- 

 garded as granitic materials which have not perfectly crystallised. 

 Mr. Ward has noticed that as the rocks severally approach towards 

 the granite masses, especially that of Eskdale, they undergo a regular 

 modification by metamorphism. First, at a distance of several miles, 

 the rock, which may have been a bed of volcanic ash, shows under the 

 microscope hazy and indistinct outlines of the fragments, and fre- 

 quently a kind of streaky-flowing structure round the larger particles, 

 though remains of original lines of bedding are observable. Nearer 

 to the granite the altered rocks put on a purple tint and develop 

 specks of mica, which are sometimes gathered into nests as the granite 



