8 



•J. A. PHILLIPS ON CONCRETION AET PATCHES AND 



Granite. Inclusion. 



Water I l^ygrometric '13 -18 



^^^^^ \ combined -59 1-25 



Silica 69-64 65-01 



Phosphoric anhydride trace trace 



Alumina 17-35 17-37 



Perric oxide 1-04 4-95 



Perrons „ 1-97 1-86 



Manganous oxide trace trace 



Lime 1-40 2-11 



Magnesia -21 1*34 



Potassa 4-08 1-82 



Lithia trace trace 



Soda 3-51 4-14 



99-92 100-03 



Specific gravity 2-72 2-73 



The results of chemical analysis are thus perfectly in accordance 

 with those which might have been anticipated from a microscopic 

 examination of the two rocks. A considerable proportion of the 

 orthoclase in the granite is, in the inclusion, represented by a soda- 

 lime felspar, probably oligoclase, while the marked increase in the 

 quantities of ferric oxide and magnesia in the latter are doubtless 

 chiefly referable to an augmentation in the proportion of mica 

 present *. 



The granite of the Cheesewring quarries, near Liskeard, is without 

 inclusions, but is traversed by veins of fine-grained granite, con- 

 taining a pinkish biaxial mica (lepidolite), in which triclinic felspar 

 is abundant. 



At Gunnislake the granite does not contain any patches pre- 

 senting definite and distinct outlines ; but wherever spots occur 

 they gradually merge into the normal rock. Sections prepared 

 from these indistinct aggregations are found to consist of a fine- 

 grained granite, in which triclinic felspar is perhaps somewhat 

 more abundant than in the surrounding rock. 



The only granite which I have examined from the county of 

 Devon is that raised at the extensive quarries of Poggen Tor, near 

 the great prison of Prince's Town on Dartmoor. This is a grey 

 granite, generally without pebble-like inclusions bounded by 

 distinct and well-defined outlines. A few patches of fine-grained 

 granite, considerably darker in colour than the surrounding rock, 

 were, however, found. The outlines of these are moderately 

 sharp, and, in one instance, a patch of this description encloses a 

 crystal of orthoclase, the angles of which are rounded, as well 

 as two distinct crystals of quartz, similarly modified. Pig. 2, PI. I., 

 represents this specimen, natural size. 



* The Eev. S, Haughton, F.R.S,, states that the second felspar in Cornish 

 granites is albite (Proc. Eoy. Soc. vol, xvii. p. 209). There are, however, pro- 

 bably exceptions to this ; and, in the present instance, the results of analysis 

 render the presence of orthoclase probable. 



