Vol. 51.] GRAlfOPHYKE AND THE GRAINSG-ILL &REISEN". 12V 



alike in the Ordovician intrusions of North "Wales and in those of 

 Tertiary age in Scotland. In Caernarvonshire the facts are specially 

 striking. 1 There the typical granophyres always carry augite, to the 

 exclusion of biotite 5 the ruder types of granopkyre and the granite- 

 porphyries contain the two minerals together ; and the true granites 

 have biotite alone. Analyses of the Charnwood Forest rocks seem 

 to prove that augite and micropegmatite can be formed together 

 from magmas possessing a wide range of chemical composition. 2 



Another remark may be made on the augite of the Carrock Fell 

 granophyres. As a rule, the length of a crystal is at most twice or 

 thrice its breadth. In the upper part of Glints Gill, however, the 

 most easterly tributary of Rough ten Gill, the augite assumes an 

 elongated needle-like habit. The same thing is seen east of Rae 

 Crags, where the needles are from 1 to 1| inch in length and less 

 than -rjij inch in diameter. Here, too, the idiomorphic felspar takes 

 on a similar acicular habit. These localities are near the boundary 

 of the granophyre. and it is interesting to compare the phenomenon 

 with the narrow blade-like habit of the biotite in other acid intrusions 

 in the Lake District. Mr. Marr and I have noted this feature in 

 connexion with marginal modifications of the Shap granite 3 and 

 the Dufton granite-porphyry. 4 It is found also in the quartz-por- 

 phyries of Potter Fell and Wansfell, and at the border of the Butter- 

 mere granophyre near Strands. The point is of interest in the 

 present instance of the Carrock Fell granophyre ; for, if the peculiar 

 habit of the augite can be safely regarded as part of a marginal 

 modification of the rock, it proves that the mineral crystallized 

 after the intrusion of the magma, and indeed — since there is no 

 parallel arrangement of the needles — after the cessation of flowing 

 movement. 



The felspars of the granophyre include scattered idiomorphic 

 crystals and the much more abundant felspathic constituent of the 

 micropegmatite. The former give sharply bounded rectangular 

 sections with albite- and pericline-lamellation, and sometimes 

 Carlsbad twinning in addition. The low extinction-angles of the 

 lamellae indicate oligoclase, but a frequent zonary banding, seen 

 between crossed nicols, shows that the crystals are not quite 

 homogeneous. 



The chief bulk of the rock consists of felspar and quartz showing 

 a great variety of ' granophyric ' structures. Under this inappro- 

 priate term ' granophyric ' Rosenbusch has included both evident 

 micrographic intergrowths of felspar and quartz (micropegmatite), 

 aualogous on a small scale to ' graphic granite ' (the original peg- 

 matite of Haiiy), and the various types of spherulitic and ' pseudo- 

 spherulitic ' structure. Recent researches have rendered it highly 

 probable that the latter are merely very minute — and perhaps some- 

 times ultra-microscopic — groupings of felspar-fibres and intergrowths 



1 See ' The Bala Volcanic Series of Caernarvonshire,' Cambridge, 1889, § iv. 



2 Berry, Quart. Journ. Geol. Soc. vol. xsxviii. (1882) p. 197. 



3 Oid. vol. xlvii. (1891) pp. 277, 284. 



4 Bid. p. 520, 



l2 



