284 ME. W. R. WATT OX THE GEOLOGY [June I9I4r 



Intermediate between these two is the contact-norite, where the 

 hypersthene has been largely eliminated and its place taken by 

 biotite. To this is added an increase in the amount of cordierite and 

 in the size of the garnets. Two felspars, at least, occur — ortho- 

 clase and andesine (/* = 1*551), the former being associated with 

 quartz. Fine intergrowths are formed by the biotite, even with the 

 cordierite. Myrmekitic intergrowths also occur of quartz and acid 

 felspar — the latter having a refractive index lower than that of 

 quartz. The specific gravity of this rock has fallen only to 2*94 : 

 as the Carvichen intrusion is much more acid than the OrdiquhilL 

 rock, this is only what might have been expected. Naturally^ 

 there is great variation in this type, and the chemical analysis and 

 calculated mineral composition may be true of only a very limited 

 type. 



In the neighbourhood of Boghead of Gribstone, Broomhillock, 

 and Ordbrae, a much fuller sequence can be made out, thanks to 

 the large number of partly-digested xenoliths. These vary from 

 a norite to a rock composed of biotite, cordierite, garnet, and fel- 

 spar, passing through, as an intermediate stage, a garnetiferous 

 cordierite-norite. 



The early stages are the most interesting, when the rock is 

 mainly composed of lrypersthene and cordierite, together with 

 garnet, biotite, and felspar. There is slight pinitization, but the 

 high specific gravity of the rock (3'17) bears witness to the small 

 amount of decomposition. The felspars occur in aggregates of 

 small grains with pleonaste and magnetite, and show symmetric 

 extinction-angles ranging up to 40° — thus indicating a felspar of 

 the composition of bytownite. The biotite is less plentiful than the 

 associated hypersthene, and shows fine intergrowths with the 

 surrounding minerals. Near Broomhillock, a very fine example 

 of a garnetiferous biotite -norite is to be obtained, wherein the 

 colourless garnets are associated with biotite, magnetite, and hyper- 

 sthene. Towards the biotite, as a rule, the faces of these garnets- 

 are well developed — towards the felspars they are usually irregular. 

 This biotite, much more abundant than in the previous case, is 

 beautifully pleochroic in red and yellow. Pleonaste, magnetite, 

 and pyrite occur with the felspars. These last are occasionally 

 labradorite (symmetric extinction-angle = 38°) and usually andesine 

 (fi = 1'551). The rock is again very compact and but little altered, 

 having a specific gravity of 3*10. 



In the second stage, the hypersthene is being replaced by the 

 biotite, and accordingly hypersthene, biotite, clear garnet, and 

 magnetite are found in close association. The biotite shows 

 numerous intergrowths with felspar, quartz, and cordierite. The 

 felspar is in places not more basic than labradorite, but accompanied 

 by it is an acid felspar forming myrmekitic intergrowths with the 

 quartz. The colourless garnet shows vermicular markings and has 

 magnetite-inclusions, while it passes into the pink variety. Pleo- 

 naste occurs as an inclusion in cordierite. 



In the following stages the hypersthene has entirely disappeared, 



