IN THE NORTH-WEST HIGHLANDS OF SCOTLAND. 387 



Archaean rocks and regional metamorphism in general, has been 

 gathered together. The chief parts of this evidence are now laid 

 before the Society. 



The lield-work of the Geological Survey in the region has been 

 executed by Messrs. Peach, Home, Gunn, Clough, Hinxman, and 

 Cadell, under the immediate supervision of Mr. Peach. Mr. Cadell 

 mapped that portion of the line of complicated structure extending 

 from the head of Loch EriboU to Loch More ; Mr. Clough, from 

 Loch More to the northern base of Glasven ; Messrs. Peach and 

 Home, from Glasven to Elphin and the Cromalt Hills ; Mr. Hinx- 

 man, from Elphin to Strath Kanaird ; and Mr. Gunn, from the 

 latter point to Little Loch Broom. 



I. Aech^an Eocks. 



The detailed examination of the Archaean rocks lying between 

 Loch Laxford and Lochinver has led to the conclusion that they 

 have been subjected to enormous mechanical movements in Pre- 

 Cambrian time. In attempting to unravel the history of these 

 ancient rocks, it is best to follow the chronological order of the 

 movements, as it enables us to interpret the successive modifications 

 which the crystalline rocks have undergone. 



1. Original Types of Gneiss, 



1. Throughout the region referred to there are certain tracts 

 where foliated rocks occur, evidently representing the original types 

 of Archaean gneiss. Erom Lochinver south to the river Kirkaig and 

 northwards along the coast to Loch Eooe, and again near Kylesku 

 by the shores of Loch A'Chairn Bhain, these typical gneisses are 

 admirably displayed. They are arranged in gentle anticlines and 

 synclines, the axes of which usually run N.N.E. and S.S.W. or N.E. 

 and S.W. Occasionally the angle of dip of the foliation is so low 

 that the outcrop of the bands forms a series of parallel escarpments 

 along the hill- slopes (Kylesku). Structurally they occur either as 

 massive, rudely foliated crystalline rocks, with few divisional planes, 

 or as well-banded gneisses in which the constituents have a distinct 

 parallel arrangement. Both varieties are traversed by segregation- 

 veins and pegmatites. The prominent minerals are plagioclase 

 felspar, pyroxene (augite, diallage), hornblende, quartz (frequently 

 opalescent), and magnetite. It is worthy of note that mica is a rare 

 constituent of these original types of gneiss. On close examination 

 it is apparent that the bands present certain lithological varieties of 

 variable thickness ; some consisting mainly of pyroxene or horn- 

 blende and a small quantity of plagioclase felspar ; some of plagio- 

 clase, pyroxene, or hornblende and opalescent quartz ; others of 

 opalescent quartz and felspar. These varieties frequently cross the 

 lines of schistosity and are evidently due to differences in the nature 

 of the materials prior to the development of the foliation *. 



* The term gneiss, as applied to these rocks, may be regarded as a misnomer, 

 but for the sake of convenience we in the meanwhile use the generally accepted 



