516 MR. A. HAKKER ON ROCKS 



and grains of felspar. Most of these have twin-laraellation and 

 often an imperfect prismatic shape, so far as their crowded occur- 

 rence permits. Others are only once twinned, and some shapeless 

 simple grains, with a tendency to occur interstitially, are perhaps 

 quartz. The structure of these patches and the limpid nature of 

 the felspars are characteristic of metamorphic rocks, and point 

 unmistakably to recrystallization of the rock in situ. Any doubt 

 is removed by an examination of the porphyritically disposed 

 felspars already noticed. These are frequently bent and broken, 

 and there seems to be some secondary twin-lamellation induced by 

 stress. The most striking feature, however, is the replacement of 

 the original turbid crystals by new felspar-substance exactly similar 

 to the little striated felspars in the general ground of the rock. 

 8ome of the original crystals do not show this alteration ; others 

 are partially changed ; and others, again, are totally converted into 

 clear crystalline aggregates, preserving only the outline of the 

 crystal from which they have been formed. The newly-formed 

 triclinic felspars, cut perpendicular to the twin-plane, give a maxi- 

 mum extinction-angle of about 18°. This agrees with albite, 

 though a certain variet}^ of andesine would give the same value. A 

 curious point about the dynamo-metamorphism is the seemingly 

 capricious manner in which it has affected the mass of the rock. 



As an example of the ordinary fine ashes of the Skiddaw Slate 

 group, a rock from Burney was sliced [921]. It shows plenty of 

 felspar-crystals, or more frequentl)'^ broken portions of crystals, 

 some showing twin-lamellation, others not. The general mass of 

 the rock has probably been a felspathic dust, but now contains a 

 quantity of quartz and calcite, besides pyrites and yellow opaque 

 spots of ferruginous matter. Of these the quartz seems to be the 

 latest-formed, occurring in little continuous patches of ragged out- 

 line and enclosing other decomposition-products. Little, if any, 

 of this mineral is in original grains, and there is no indication of 

 detrital material mingled with the volcanic. 



The grits in this group of rocks are derived from the disintegra- 

 tion of igneous rocks of more than one kind. A specimen from the 

 north side of Mudgill Sike [964] was found to consist essentially of 

 grains of quartz, unrolled crystals of plagioclase, and rather rounded 

 fragments of a microlithic andesite. A few rolled granules of 

 decomposing augite occur, and a fragment of quartz-porphyry 

 showing a microgranitic groundmass and a portion of a porphyritic 

 (juartz. The quartz -grains, which are the most abundant consti- 

 tuent, are mostly subangular to rounded, but some of the smaller ones 

 are quite angular. They are for the most part clear, though some 

 contain rows of fluid-pores. The felspars are sometimes j)enetrated 

 by apatite needles, and resemble those which occur porphyriticaU}' 

 in many andesites. In sections perpendicular to the albite twin- 

 plane the extinction-angles range up to 13" or 14°. The rock con- 

 tains a little calcite, partly in pseudomorphs after felspar. Other 

 authigenetic constituents are pyrites and a little quartz, the latter 

 sometimes forming narrow veinlets. 



