HARKER : NOTES ON NORTH OF ENGLAND ROCKS. 241 



which gives low polarisation-colours (neutral tints and indigo), and 

 only partially preserves the form of the original flakes. 



The ground-mass consists of a finely granular aggregate of quartz 

 and felspar, the latter preponderating. The quartz is for the most 

 part of anterior consolidation to the felspar, and forms clear grains 

 of irregular shape imbedded in the later mineral. Calcite dust, 

 giving bright red and green polarisation-tints, is among the 

 decomposition-products of the felspar, indicating that that mineral 

 contained a certain proportion of lime. Specimens from Shundraw 

 [753] and Threlkeld [805] show identical characters. Mr. Hughes' 

 analysis of the St. John's Vale rock seems to indicate about 24 per 

 cent, of quartz, 16 of orthoclase, 40 of a felspar near oligoclase, and 

 20 of decomposition-products of felspar and mica. It is evident, 

 therefore, that the felspar of the ground-mass must be in part of 

 a soda-lime variety, or perhaps it may be a felspar containing both 

 potash and soda. 



(xi) Granite, etc., of Eskdale and IVastdale, Cumberland. — The 

 normal type in this intrusion or set of intrusions is a granite of 

 moderately coarse grain with red felspar, grey quartz, and dark mica. 

 A slice from Stanley Gill [747] shows some interesting features, the 

 most striking being the variety of acid felspars present and their intri- 

 cate intergrowths with one another. Ordinary orthoclase is seen, and 

 oblong crystals of a plagioclase of the albite-oligoclase series showing 

 close twin-striation and low extinction angles. Besides these, we 

 observe broad crystal plates showing a minute 'cross-hatched' structure 

 between crossed prisms. This appearance, which is best seen when 

 the crystal is nearly in the position for extinction, is distinctive of 

 microcline, the triclinic potash-felspar. Again, the orthoclase is seen 

 in many cases to be traversed by little irregular veins of another 

 felspar, doubtless albite, which extinguishes between crossed Nicols 

 in a rather different position from the orthoclase. All the little veins 

 within one crystal extinguish simultaneously, and indeed the albite is 

 intergrown with the orthoclase in a definite crystalline relation to its 

 host. This is the so-called 'micro-perthite.' A precisely similar veining 

 with albite is seen in some of the microcline crystals in the same slide. 



All the felspars are partially decayed, and their turbid appearance 

 contrasts with the quartz, which is the next important ingredient of 

 the rock, in grains filling the interstices between the felspar crystals. 

 The quartz, however, contains very numerous minute fluid-pores, 

 which are mostly arranged in parallel lines in each grain. The 

 remaining constituent is biotite, which, however, is highly altered by 

 secondary changes. Much of the mineral is rendered black and 

 opaque b y impregnation with iron-oxides derived from its own 



August 1890. (J 



