and Igneous Rocks of Ensay. 103 



pegmatite veins were formed, in connection with metamorphic 

 action. The great constancy with which these irregularities 

 of structure occur in felspars of this kind in different 

 localities, shows, moreover, that the processes of formation 

 have also some degree of uniformity in their action. 



I carried out a quantitative analysis of this sample, with 

 the following results : — 



No. 9.- 



— MlCROCLINE - PeRTHITE . 





Si.O„ ... 





6355 



A1. 2 3 ... 



... ... 



20-36 



Fe. o 3 ... 





•tr 



Ca.O ... 





•35 



Mg.O ... 



... • . . 



•20 



K 2 ... 



... ... 



12-00 



Na. 2 ... 





352 



H o ... 



... ... 



•52 



100-50 



Hygroscopic moisture ... ... *31 



Sp. grav 2-573 



The Mg.O and some of the H o in the above can be 

 referred to talc, a few flakes of which occur in the sample. 

 Some of the combined water belongs to kaolinised parts of 

 the felspar. Disregarding these extraneous constituents, the 

 remainder can be calculated out as potassa, soda, and lime 

 felspars, in the molicular proportions of 2'040, *904, and -052 

 respectively. The last probably represents the third felspar, 

 which I have mentioned as being determinable in the thin 

 slices. Assuming it to be an oligoclase, and to have a normal 

 constitution — for instance, of Alb. 3 to An. 1 — I may then 

 say, with some reasonable probability of being not far from 

 the truth, that this microcline-perthite is composed of 

 microcline, albite, and oligoclase, in the proportions of 

 10 : 3*6 : 1* nearly; or, taking the two latter felspars together, 

 the proportion between microcline and albite, + oligoclase, 

 would be nearly as 2 : 1. 



This fairly agrees with the mental conception which I 

 have formed by an inspection of the thin slices under the 

 microscope. The sketches given in Plate IV. differ from this 

 in so far that, as I intentionally selected a part of each slice 



