Vol. 51.] GRANOPHYRE AND IHE GRAINSGILL GREISEN. 141 



mica has become an essential constituent, occurring frequently in 

 parallel intergrowth with the brown. Besides this mica, clearly 

 original, there are in places many minute scales developed in the 

 interior of the felspar-crystals in a manner suggestive of secondary 

 change. Part of the quartz is enclosed by orthoclase, or tends to 

 form with it a rude graphic intergrowth. 



In the road, less than 100 yards farther north, the white mica is 

 seen to be somewhat more abundant, but the rock is too much 

 decomposed for examination. Going northward up the hill which 

 occupies the angle between Grainsgill and the Caldew, we see no 

 more exposures for nearly 100 yards, during which the character of 

 the rock has completely changed. Biotite has disappeared, and the 

 rock is seen to be composed essentially of quartz and white mica. 

 The microscope shows that a little orthoclase still occurs, though in 

 very subordinate quantity. Much of the mica is not strictly 

 colourless in sectious, but has a yellowish tinge with faint pleo- 

 chroism. The mineral occurs in two ways : firstly, as flakes of some 

 size, but of rather irregular contour and often moulding the quartz 

 and felspar ; and secondly, as densely packed little scales, which, 

 with quartz, form more or less defined patches, and may be imagined 

 to represent felspar-crystals. The bulk of the quartz occurs as an 

 irregular mosaic traversed by subparallel rows of fluid-pores, which 

 contain bubbles and sometimes minute crystals [1887]. 



On the crest of the slope the rocks have the same general cha- 

 racter. Felspar is quite subordinate and sometimes wholly wanting, 

 while the light mica is very plentiful. It occurs in relatively large 

 flakes, in fan-like, sheaf-like, or feathery groupings of smaller flakes, 

 or in aggregates of minute scales, and is very pale to colourless in 

 sections. Much of the rock exposed here has a porphyritic aspect, 

 due to the development of large grains of quartz. In another type 

 the quartz-mosaic becomes rather coarse-textured, the mica occurs 

 only in rather large flakes, and the rock has more of the character 

 of a true greisen. My friend Mr. L. J. Spencer, now of the British 

 Museum (Natural History), has had the kindness to analyse this 

 rock, and his results are given below, side by side with Mr. Hughes's 

 analysis of the normal Skiddaw granite : — 



IV. V. 



Si0 2 75-223 80 36 



A1 2 3 11-140 11-12 



Pe 2 3 trace 1 -. ~„ 



FeO 1771 J i " 



MgO 1-081 056 



CaO 1-624 067 



Na 2 3-996 1-82 



K 2 4-516 247 



Loss on ignition ... 0500 1-96 



PA 0-149 — 



100-000 100-73 



IV. Skiddaw granite, White Gill (see Ward, Q. J. G. S. vol. xxxii. 1876, p. 5). 

 V. Greisen, eastern slope of Combe Height, 250 yards south of Grainsgill 

 Lead-mine : sp. gr. 2 - 684. 



Q. J. G. S. JS T o. 202. M 



