19 



Harker and Marr as occurring in the Shap Fell granite, ^12) 

 but in the present instance the dark fine-grained rock, though 

 of higher specific gravity, has actually a slightly higher per- 

 centage of silica than the main granite. 



(3) EVEN-GRAINED GRANITE. 



This rock is everywhere uniform in type. Microscopically 

 it is of medium variable grainsize, and for the most part 

 typically allotriomorpliic granular, though a little graphic 

 intergrowth is noticed. The mineral constitution is difficult to 

 determine quantitatively, owing to the very uneven distribu- 

 tion of the minerals through the rock mass. The following 

 results were obtained from measurements on three different 



slides : — 0/ 



/o 



Quartz 426 



Microcline ... ... ... ... 42'7 



Plagioclase ... ... ... ... 86 



Biotite 61 



There are also a few granules of sphene and apatite and 

 some tiny zircons. A little muscovite is intergrown with the 

 biotite, and a small amount of secondary pyrites fills cracks 

 between quartz grains. The rock shows evidence of strain, 

 slight granulation and wavy extinction being present. Alter- 

 ation is fairly marked : the felspars are cloudy and the biotite 

 is changing to chlorite. 



Quartz is granular and is- much cracked. It contains 

 strings of tiny inclusions, many of which are glass-cavities in 

 the form of negative crystals. A few liquid inclusions were 

 also observed, but the peculiarities noted in the blue quartz 

 of the normal granite are absent. Microcline is distinguished 

 by its characteristic twinning; traces of cryptoperthitic inter- 

 growth were detected. The mineral is very much kaolinized. 

 The plagioclase is zoned, but owing to alteration the composi- 

 tion was hard to determine. It appears, hoM^ever, to grade 

 from oligoclase at the centre to albite (Ab,,.) at the 

 periphery. Alteration is very marked to calcite and sericite. 



The crystals often exhibit a highly altered kernel sur- 

 rounded by a shell of clear fresh material. One felspar 

 crystal consists of plagioclase surrounded by a broad con- 

 tinuous zone of fresh potash felspar, A single very tiny 

 crystal of tourmaline was detected among some granulated 

 quartz. 



Primary iron ores are virtually absent, but a little 

 secondary haematite is present. Tlie biotite does not call for 

 remark. 



(12)Q.J.G.S., vol. 47 (1891), pp. 280-282. 



