286 MESSRS. A. HARKER AND J. E. MARK ON 



The hig^hest sill, which, owing to the clip, is the lowest down the 

 valley, shows a grey compact ground, studded witli little quartz- 

 grains and Hakes of dark mica, and enclosing porphyritic felspars, 

 some of which are one or two inches long. The quartz grains are 

 mostly rounded, but occasionally a bipyramidal crystal is seen. 

 The most conspicuous feature of the rock is the occurrence of 

 •felspars of flesh-red colour, some with Carlsbad twinning, which at 

 once recall those of the Shap Fell granite, but have the rounded 

 outlines and often the well-marked borders associated especially 

 with the dark inclusions in that rock. Besides the above minerals, 

 the thin slices cut from this sill [1157, 1158] contain apatite 

 prisms, occasional zircons, and abundant acute-angled crystals of 

 brown pleochroic sphene, like those so characteristic of our granitic 

 inclusions. AVhen the zircon is enclosed by mica, it is surrounded 

 by an intensolj- absorbent pleochroic halo — a character which we 

 have noted in the Shap Fell granite itself, and which is well known 

 in many others. The mica is of the usual brown colour. Its mode 

 of alteration is sometimes like that of the mica in the granite ; 

 while sometimes it gives rise to the interpositiou of lenticles and 

 streaks of calcite along the cleavage-lamella) in the fashion usually 

 seen in lamprophyric rocks. The rounded grains of clear quartz 

 have inlets and enclosures of the groundmass, which is that of an 

 ordinary quartz-porphyry, in which, however, part of the felspar 

 has separated out in little prisms. The porphyritic felspars enclose 

 a few mica-flakes, as well as the earlier accessories. Both ortho- 

 clase and oligoclase are represented. The latter sometimes occurs 

 in clusters of small crystals, with irregular junction wdth one 

 another, but presenting crystal forms to the surrounding groundmass. 

 Mr. Teall *, in describing similar clusters of felspar crystals in the 

 Tynemouth dyke, has pointed out that this accords with the view 

 that such crystals were formed under plutonic conditions, and 

 floated up in the magma into their present position. 



The next sill has, to the eye, a similar grey ground, with perhaps 

 rather more mica, and encloses little plagioclase crystals and 

 scattered grains of quartz about jLinch in diameter, but apparently 

 none of the large red felspars. The microscopic characters accord 

 with those of the former rock, except that there is very little sphene 

 present [1159]. 



The preceding rocks may be called micaceous quartz-porphyries. 

 The next sill has in the field a dull brown ground crowded 

 with flakes of brown mica, and would naturally be mapped as 

 a mica-trap. It contains, however, large red felspars with rounded 

 outline, and a few scattered blebs of quartz. The microscope shows 

 that these felspars are of a striated variety, probably near oligoclase, 

 with a narrow border of orthoclase [1160], The interior of each 

 crystal is twinned according to the albite and Carlsbad laws, and 

 the Carlsbad twinning is continued into the border of orthoclase. 

 The slide contains abundant brown mica, which has sufl'ercd altera- 

 tion chiefly of the kind producing calcite : there is but little mag- 

 * Geol. Mag. (1889) p. 481. 



