18 E. Wynne Hughes — Geology of 



IS. 40° E. and dip N. 50° W. at an angle of 60°, being thus roughly 

 at right angles to the major joints. 



In traversing the hill in a south-easterly direction from the quarry 

 several rock exposures are met with. A few yards away from the 

 quarry a dark rock, containing porphyritic crystals of quartz and 

 pink felspar, is seen once more. It is in every respect macro- 

 scopically similar to the dark f elsitic rock mentioned above. We thus 

 have a band of pale f elsitic rock (which we may term the pale 

 variety) lying between two dark f elsitic bands (which we may term 

 the dark variety). The more southerly band is about 40 feet wide. 

 Following is a narrow band of the pale variety. Succeeding this 

 a dark felsitic rock again occurs as a band about 20 feet wide. 

 Macroscopically it only differs from the dark bands in that the pink 

 felspars are distinctly smaller. This band is succeeded by another of 

 the pale variety — 15 feet thick — and in turn by a massive dark band, 

 macroscopically very similar. 



Proceeding eastwards from this point, there is no exposure for 

 fully 100 yards until another small quarry is reached. From here 

 on to the most easterly part of the hill, igneous rock is exposed at 

 several places, but in all cases it consists of the pale variety, 

 macroscopically similar to bands ahead}' noticed in the first quarry. 



Along the south-west flanks of the hill a similar alternation of the 

 pale and dark rocks is evident. These bands, however, are not 

 persistent. For instance, the thin band does not occur on this side, 

 so that two bands have here coalesced, giving a massive band of the 

 dark variety having a width of fully 100 feet. Further south occurs 

 also a tongue of the dark variety wedging into the pale variety. 

 The line of strike given by this rough banding of the two varieties 

 of rock is B". 60° E. 



(ii) Microscopic Description. — On examining thin sections of the 

 rock from the various exposures there is found to be a surprising 

 resemblance between the pale and the dark varieties. 



(a) Phenocrysts. — In both cases porphyritic quartz and felspar 

 crystals are prominent, the quartz being the more abundant. The 

 quartz sometimes show good crystal forms, but more often are 

 corroded and rounded (PL I, Figs. 1, 2), and frequently the matrix 

 may be seen in cracks in the crystals (PI. I, Figs. 1 and 5). 

 Most of the porphyritic crystals of felspar are plagioclases with 

 multiple twinning (PI. I, Fig. 5), but in nearly all cases simply 

 twinned or untwinned crystals of orthoclase are also present (PI. I, 

 Fig. I). The plagioclases appear to approximate to albite, and 

 crystals of chequer albite 1 are occasionally present. This chequer 

 albite is sometimes intergrown in perthitic fashion with untwinned 

 felspar. Phenocrysts of biotite are also sparingly distributed 

 throughout the rock. Muscovite, evidently of secondary origin, is 

 always present. 



(b) Groundrnass. — These various phenocrysts are embedded in 

 a felsitic groundrnass. This again shows a general similarity in both 

 the pale and the dark varieties, although a certain amount of 



1 J. S. Flett, Mem. Geol. Surv. (Newton Abbot), 1913, p. 60. 



