520 MR. A. HARKEK ON ROCKS 



Fell granite*. The whiter and finer-grained rock which crops out 

 just to the eastward of the foregoing is probably only a marginal 

 modification of the main mass. Here the plates of white mica are 

 rarer, and the most striking feature is the occurrence of long narrow 

 blade-like crystals of dark mica, precisely like some found on the 

 margin of the Shap Fell granite and in the dyke on Potter Fell, 

 which seems to be connected with that massf. 



A slide [842] of this Dufton Pike rock shows plenty of porphyritic 

 quartz, in clear idiomorphic crystals with only a few glass- cavities 

 or small inclusions of groundmass. Among the porphyritic felspars, 

 a plagioclase with Carlsbad-, albite-, and pericline-twinning pre- 

 dominates. The light mica is perfectly clear and colourless, the 

 dark decomposing with a green colour. Earely the two are inter- 

 grown. Both micas recur in small flakes with a rough parallel 

 disposition, and these must be regarded as part of the groundmass. 

 An occasional hexagonal prism of apatite is seen. The ground of 

 felspar and quartz is of the microcrystalline or " microgranite " 

 type. 



A specimen from a dyke north-west of Cuns Pell differs somewhat 

 from the preceding, especially in the absence of white mica, and 

 probably represents the usual type of the district [918]. The 

 porphyritic crystals of quartz have their edges rather rounded, and 

 are sometimes broken, but the fragments are not far separated. A 

 fl.ake of dark mica is sometimes enclosed in the quartz, as well as 

 in the porphyritic felspars. 



On the hillside north-west of the " Spring " in Ousby Dale the 

 rock shows some remarkable modifications, which can be referred 

 only to intense dynamic metamorphism. In the field it shows only 

 a slightly different appearance from other examples of these quartz- 

 felspar-porphyries, except that it has a general yellow iron-stained 

 colour. A section, however, shows that a large part of the rock 

 consists of colourless mica [1319]. The porphyritic felspars without 

 losing their form are completely replaced by minute scales of this 

 mineral, the scales in any one pseudomorph having a very general, 

 though not uniform, orientation parallel to the length of the 

 original felspar-crystal. Similar scales of mica occur in great 

 quantity in the general mass of the rock, together with large flakes, 

 which are rather ragged and wav}', and do not give very precise 

 extinction between crossed nicols. The rounded and corroded por- 

 phyritic crystals of quartz are only occasionally cracked and broken. 

 Dark mica is absent, but is perhaps represented by the larger 

 flakes of colourless mica, each of which encloses a shapeless 

 patch of limonite. Little flocculent patches of yellow ferruginous 

 matter occur also in the groundmass. Except for these and the 

 minute scales of mica, the ground consists of a clear microcrystalline 

 mass of quartz and felspar, without trace of crystal outlines, and 

 highly suggestive of recrystallization m situ. It is not easy to 



* See p. 288 of this volume, 

 t See p. 277 of this volume. 



