V.—On certain Gneisses with Round-grained Oligoclase and their Relation to Pegmatites

Abstract

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 other 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 rounded grains of oligoclase.