PROCEEDINGS OF THE COTTESWOLD CLUB 245 



minerals are at first broken up into simpler compounds, 

 and these products often form new combinations. Take 

 for example, the change frequently undergone by potash- 

 felspar. This is a silicate of alumina and potash. In the 

 metamorphism, a portion of the silica separates as quartz, 

 and the remainder, in union with the alumina and potash, 

 forms white mica. Apply this change to the metamorph- 

 ism of the Malvern granite. This granite is a coarse 

 crystalline compound of quartz and a reddish potash- 

 felspar. The quartz remains unchanged. A part of the 

 felspar breaks up into quartz and white mica ; and the rest 

 is reconstructed in granules, or in small crystals. Thus 

 a binary granite, consisting of quartz and felspar confusedly 

 intermixed, is transformed under pressure into a rock 

 made up of quartz, felspar, and mica, with these minerals 

 arranged in lenticular layers, so as to form a mica-gneiss. 



Sometimes in the crushing process a portion of the 

 rock proves too tough for the earth-mill, and remains 

 unaffected. Owing to the shearing motion of the sur- 

 rounding mass, this core is made to assume a lenticular 

 form, while the planes of sliding curve round it, just as the 

 current in a rivulet is divided by a boulder in mid-stream, 

 and bends round it on each side. These lenticular cores 

 are called " eyes," and the rock containing them is known 

 as "augen-gneiss." 



The activity of the chemical forces in the rock will 

 obviously be greatlv increased by its crushed condition. 

 A crushed granite is almost as porous as a sponge. The 

 products of the decomposition of a diorite, such as iron- 

 oxide and chlorite, can often be traced into an adjoining 

 sheared granite for many yards, passing between lenticular 

 flakes and filling microscopic cracks. In this way, heated 

 waters containing in solution alkalies, alkaline carbonates, 

 and other chemical re-agents, can pass from place to ])lace, 

 and take an active part in the production of new mineral 

 compounds. 



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