64 George Barrow — On certain Highland Gneisses. 



luay perhaps permit me some time to print. It was in my view by 

 a combination of these two movements tbat the diluvial phenomena 

 to which I refer were jDi'oduced. This cause is at all events an 

 efficient one, and is therefore not like so many of the pbysical causes 

 appealed to by the current school of Uniformity, both inefficient 

 and transcendental. 



V. — On certain Gneisses with Eound-grained Oligoclase and 



THEIR EeLATION TO PEGMATITES. 



By George Barrow, F.G.S., 

 H. M. Geological Survey. 



[Communicated by permission of the Director-General.] 



IN the course of my work in the Highlands of Forfar, I have 

 been much struck with the mode of occurrence of certain 

 light-coloured gneisses, of undoubted igneous origin. 



They are intruded into the surrounding rocks in an infinite 

 number of thin bands or sills, generally interlacing, and often 

 not more than two feet thick ; sometimes not exceeding an inch. 

 Their bulk in some areas exceeds considerably that of the older 

 rocks, at otlier times it is far less. 



Two points are easily noted ; first, they have no selvage edge ; 

 secondly, they have a very characteristic aspect, due to I'ounded 

 grains of oligoclase. 



Commencing in an area where these characters are well marked, 

 we find that the rock consists of oligoclase, muscovite, biotite, 

 quartz, and microcline, but the last mineral bulks far less than the 

 more basic felspar. It is the oligoclase that is so round-grained in 

 form, and gives the rock its characteristic appearance. The micas, 

 especially the muscovite, may easily be seen to have sharp angles. 



Going southwards, that is, away from an area where gneisses 

 predominate, we obsei've the round-grained chai'acter to become less 

 marked, and the oligoclase to form a smaller proportion of the 

 whole rock. Still further south, the gneissose character becomes 

 less marked, the oligoclase is further diminished in amount, while 

 muscovite begins largely to exceed the biotite. 



In this phase the rock begins to be much permeated with coarse 

 pegmatite, which forms a massive fringe to the southern edge of 

 the gneiss, this fringe attaining in one case a breadth of 700 

 yards. The pegmatite consists essentially of microcline, quartz, and 

 muscovite, the oligoclase being usually very small in quantity, and 

 not visible to the unaided eye. That this rock consolidated much 

 as we now see it is obvious from the vast area of contact meta- 

 morphism that accompanies its intrusion. How then was it pro- 

 duced? I learned the solution of the enigma from a normal granite. 

 A common type of granite consists of potash-felspar, quartz, oligo- 

 clase and two micas; the first-named mineral usually attaining the 

 greatest size, and being most striking to the eye. But examine 



