THE SHAP GRANITE AND ASSOCIATED KOCKS. 281 



higher than the sp. gr. of the normal granite, and the difForcnee is 

 greater than in !>Ir. Phillips's rocks *. 



Matrix. Inclusion. 



Greadv 2-72 2-73 



Petorliead 2-69 2-73 



Ardshiel (Fort William) . . — 2-93 



Shap Fell 2-687 2-769 



It is interesting to compare the dark patches in the Shap Fell 

 rock with those described by Dr. Ch. Barrois f in the intrusions 

 near Rostrenen in Britanny. This, too, is a biotite-granite with 

 large porphyritic crystals of orthoclase, which, however, are not red 

 but white. In it occur patches of darker colour than the normal 

 type, containing less orthoclase in the groundmass, more plagioclase, 

 and that of a more basic variet}", and more apatite. In these patches, 

 however, the porphyritic crystals of orthoclase are wanting. This 

 seems to be explained by the fact that these crystals, unlike those 

 of the Shap rock, are of rather late consolidation, being posterior to 

 the mica. In the Eostrenen rock, too, the patches are described as 

 graduating into the normal rock, which would seem to indicate a 

 greater degree of fluidity at the time of injection than in the case of 

 the Shap granite. 



Microscopic examination shows that these dark patches differ in 

 some respects from the normal granite of the quarries, in both the 

 relative proportions and the arrangement of the constituent minerals 

 (see PL XI. fig. 2). 



Apatite occurs rather plentifully, though locally, in small clear 

 needles. 



Zircon is less abundant, but a few crystals occur, chiefly in the 

 mica. 



Magnetite is present rather sparingly in little crystals and grains, 

 as in the normal granite. 



Sphene and dark mica occur in much greater abundance than in 

 the typical Shap granite, and it is the latter mineral which gives 

 the prevailing dark colour to the patches in question. It is mostly 

 in rather small flakes, and shows much of the green decomposition- 

 product noticed above. The sphene is sometimes almost as plentiful 

 as the mica ; it forms acute-angled crystals, as already described, 

 ■or rounded grains, with deep brown colour and strong pleochroism. 



The felspars here are almost constantly idiomorphic, and besides 

 the porphyritic orthoclase, to be separately noticed, occur in larger 

 and smaller crystals. Among these the triclinic felspar predominates 

 over the monoclinic, and is more abundant than in the normal 



* Among foreign rocks, the biotite-granite of the Barr-Andlau district in the 

 Vosges compares very closely with that of Shap Fells. The figures given bv 

 Rosenbusch (' Steiger Schiefer,' pp. 147, 154, ed. 1877) are : — 



Matrix. Inclusion. 



Silica-percentage 68-967 57*894 



Specific gravity 2-680 2-779 



t Ann. Soc. Geol. du Nord, vol. xii. (1885) p. 6. 



Q. J. G. S. No. 187. X 



