Vol. 50.] ACID ROCKS OF THE INXER HEBRIDES. 219 



the base of the cliffs shows that the vertically of the junction- line 

 continues southward. The same relation can be followed on the 

 opposite side of Strath na Creitheach and along the precipitous front 

 of Grarbh-bheinn. 



The rock of Meall Dearg and of the declivities lying to the south 

 of that eminence displays the drusy, micropegmatitic structure so 

 characteristic of the granophyres of the Inner Hebrides. But one 

 new feature of interest has been observed in it which deserves 

 mention here. While examining under the microscope some thin 

 sections of this rock, Mr. Teall has discovered that it contains the 

 mineral riebeckite. At my request he has been so good as to draw 

 up the following note on the subject: — ■ 



"The rock is medium-grained, light-coloured, and contains drusy 

 (miarolitic) cavities. The principal constituent is a micropegmatitic 

 intergrowth of quartz and orthoclase, but more or less idiomorphic 

 crystals of the same minerals occur. The felspathic portion of the 

 micropegmatite, which usually surrounds the idiomorphic orthoclase, 

 extinguishes simultaneously with the central crystal. There are 

 small irregular patches, opaque or nearly so by transmitted light, 

 and brown by reflected light. These may represent in some cases 

 corroded biotite. Extremely thin opaque black plates (? ilmenite) 

 and riebeckite are the only other minerals present, and these appear 

 sparingly. 



" The riebeckite occurs in the well-known spongy forms, and 

 sometimes as idiomorphic crystals, or rather as crystals which are 

 idiomorphic in the prismatic zone. The usual forms are those of 

 the prism {110}, but in one case the cliuo-pinacoid { ( '10} was also 

 observed. The a axis is most nearly coincident with the vertical 

 axis, and the pleochroism is as follows : — 



a and /3 deep blue ; 

 y greenish brown. 



" Sauer gives green as the colour from rays vibrating parallel to y, 

 and Harker brown. The determination of the colour in this case 

 would probably vary with different observers and with the thickness 

 of the section ; but there can be no doubt as to the presence of an 

 olive-green tinge in this mineral when viewed by rays vibrating 

 parallel with y." 



As may be commonly observed among intrusive rocks of all ages and 

 compositions, the granophyres of the Inner Hebrides become finer- 

 grained towards their margin. They assume sometimes even a felsitic 

 texture and exhibit flow-structure and well-developed spherulites. 

 These characters occur altogether independently of the nature of the 

 adjoining rock. Thus in Mull they may be seen where the acid 

 rock impinges upon the bedded lavas of the plateaux and upon the 

 intrusive gabbros. In Raasay they are well exhibited where the 

 protrusions have taken place among Jurassic sandstones and shales. 

 Such well-known phenomena of contact are often of service in 

 distinguishing truly intrusive rocks. 



Q2 



