258 MR. R. H. RASTALL ON THE [May I906, 



III. Petrographical Descriptions. 

 (a) The Normal Rock. 



The macroscopic appearance of an average specimen is that of a 

 rather fine-textured granite, usually of a bright pink colour, but 

 sometimes grey with only a slight tinge of red. Near the margin it 

 is often grey or even greenish, owing to the higher proportion of 

 coloured minerals ; and in certain localities there are also varieties 

 which are quite dark-grey or blue, but these demand separate de- 

 scription. The rock is very rarely porphyritic to the unaided eye, 

 and dark-coloured ' basic patches ' are also scarce. In places the 

 rock is somewhat drusy, and in the neighbourhood of planes of 

 disturbance it is often much stained by haematite. 



I have examined a large number of slices, which have been 

 selected, as far as possible, so as to illustrate the different types, 

 and the variations that occur in different parts of the mass. 



The rock consists essentially of quartz, various felspars, chlorite 

 representing ferromagnesian minerals, and accessories. The 

 coloured minerals generally occur in small proportion only ; and, on 

 the whole, the rocks are distinctly leucocratic in character. 



Quartz is generally very abundant. It contains the usual fluid- 

 inclusions, which are nearly always very conspicuously arranged in 

 lines ; and these lines are often appreciably parallel throughout the 

 slice, as if their position was determined by strains set up during 

 cooling or subsequently. The relations between the quartz and 

 the felspar, with regard to age, are very variable ; in the granitic 

 types [4748-50, 4752] the quartz appears to have crystallized 

 partly before the felspar, as in the Shap Granite and parts of the 

 Eskdale Granite. In this case, the quartz occurs in small crystals 

 which sometimes show fairly-good hexagonal outlines, but more 

 often they are more or less rounded in form. In the great mass of 

 the rock, however, the quartz and the felspar have crystallized 

 simultaneously, giving rise to more or less perfect granophyric 

 structure [4755-56, 4758-59]. 



The felspars are very variable, and include orthoclase, perthite, 

 and plagioclase ; in a few specimens a very little microcline 

 appears to be present. The relative proportions of orthoclase and 

 plagioclase vary greatly, but the latter is generally the more 

 abundant ; in some slices it appears to be almost the only felspar 

 present. Both kinds of felspar are generally somewhat decomposed, 

 being partly converted into aggregates of small flakes of white mica, 

 and these flakes generally lie parallel to the cleavages of the felspar : 

 wherefore, in most sections, they appear to be arranged in two sets 

 at right-angles. However, the decomposition has rarely gone far 

 enough to mask the twinning, which is generally conspicuous. 



In spite of decomposition, it is often obvious that much of the 

 unstriated felspar possesses poorly-developed perthite-structure, and 

 the so-called 'orthoclase ' must contain a good deal of the albite- 

 molecule. 



