NEIGHBOURHOOD OF ST. MINVER, CORNWAIJ.. 



399 



tiary Age in Scotland and Ireland," pi. v. figs. 1, 3, and 5 ; and Prof. 

 Oreen has lately shown me some good examples from Ehobell Fawr 

 occurring in rocks, probably of Llandeilo age, and from another spot 

 two miles E.N.E. of Dolgelly, intrusive in Lingula-flags. 



Taking certain parallel lines as traces of cleavage-planes, parallel 

 to the vertical axis, we get the maximum extinction in some of the 



Fig. 3. — Augite-andesite from Oarlion, near St. Minver, Cornwall, 



tv ?•• - v; 





The black labyrinthine patch in the middle of the drawing is ilmenite ; 

 whether this patch consists of a single crystal or of an aggregate of crystals 

 of ilmenite is uncertain. The remainder of the drawing represents triclinic 

 felspars (more or less altered), felsitic matter, augite, and serpentine. 

 Specimen no. 7, X 25. Ordinary substage illumination. 



augite sections in the Carlion rock at an angle of 39° from this 

 direction. In other sections the angle varies, but usually ranges 

 from 30° to 40°. Tested with a single nicol, the thin sections of 

 this mineral show little or no appreciable trace of pleochroism. 

 These crystals depolarize in vivid colours, and, when seen by ordi- 

 nary transmitted light, appear of a pale greenish tint. It is also 

 noteworthy that in sections of this rock the ilmenite frequently 

 presents a similar labyrinthine appearance, as shown in fig. 3 ; but 

 owing to the opacity of this mineral and the irregular outlines which 

 these ilmenite patches present, it is difficult to say whether we are 

 dealing with one crystal or with many. 



The rock itself is an augite-andesite * (so far as its mineral con- 



* Many petrologists may object to the application of the name andesite to 

 this rock, since it is holocrystalline ; but if, in defining an andesite, we admit and 

 require the presence of a ground-mass, we then have two terms for one rock, 



