and Igneous Rocks of Ensay. 97 



These rocks are clearly quartz-mica diorites, and also the 

 first of what I may call the normal intrusive rocks of the 

 district, which I have had to describe in following this section 

 from Ensay. 



The aplite veins which traverse these rocks are reddish in 

 colour, and rather compact in appearance, showing in places 

 a little grey-coloured quartz. They are themselves traversed 

 by very small veins, almost mere partings of epidosite. 



The felspar of the sample which I examined is mainly 

 orthoclase, in large irregularly-formed masses rather than 

 crystals, including quartz grains. This felspar is orthoclase. 

 A second felspar is in few small and broken crystals, many 

 of which are much wasted. There is a very little magnesia- 

 iron mica, and the remainder of the rock is made up of 

 quartz grain s. 



I made a quantitative analysis of this rock for comparison. 



No. 7. — Aplite. 



Si.0 2 75-74 



Al. a O„ ... ... ... 12-45 



Fe. J 3 ... ... ... 1-02 



Ca.O ... ... ... 1-00 



Mg.O ... ... ... -08 



K 2 .0 6-77 



Na 2 .0 ... ... ... 2-91 



H 2 .0 ... ... ... -33 



100-30 



Hygroscopic moisture ... .47 



Sp. grav. ... ... ... 2*635 



In calculating the mineral percentages of this rock, I have 

 kept in view the small amounts of epidote unavoidably 

 included in the sample, the traces of magnesia-mica, and the 

 hydrated iron ore which colours the rock. Allowing for 

 these, there remain only constituents of the felspars, the 

 surplus Si.0 2 representing free quartz. 



On this basis a calculation is practicable which gives a 

 result of felspar to quartz as ... ... <*. 2* to 1" 



Or, Orthoclase 41'80 



Albite 25-72 



Quartz, 32-48 



100-00 



