616 REPORT— 1901. 



origin ; (2) a great series of igneous rocks intrusive in the fundamental complex in 

 the form of dykes and sills.' 



The rocks of the fundamental complex which have affinities with plutonic 

 igneous products occupy the greater part of the tract between Cape Wrath and 

 Skye, Mr. Teall has siiown that they are essentially composed of minerals that 

 enter into the composition of peridotites, gabbros, diorites, and granites ; as, for 

 example, olivine, hypersthene, augite (including diallage), hornblende, biotite, 

 plagioclase, orthoclase, microcline, and quartz. In 1894 he advanced a classifica- 

 tion of these rocks, based mainly on their mineralogical composition and partly 

 on their structure, which has the great merit of being clear, comprehensive, and 

 independent of theoretical views as to the history of the rock masses. Stated 

 broadly, the principle forming the basis of classification of three of the groups is 

 the nature of the dominant ferro-magnesian constituent, viz., pyroxene, horn- 

 blende, or biotite, while the members of the fourth group are composed of ferro- 

 magnesian minerals without felspar or quartz - The detailed mapping of the 

 region has shown that these rock-groups have a more or less definite geographical 

 distribution. Hence the belt of Lewisian gneiss has been divided into three dis- 

 tricts; the first extending from Cape "Wrath to Loch Laxford ; the second, from 

 near Scourie to beyond Lochinver, and the third from Gruinard Bay to the island 

 ofllaasay. In the central area (Scourie to Lochinver) pyroxene gneisses and 

 ultrabasic rocks (pyroxenites and hornblendites) are specially developed, while the 

 granular hornblende rocks (hornblende gnei?s proper) and the biotite gneisses are 

 characteristic of the northern and southern tracts. These are the facts, whatever 

 theory he adopted to explain them. 



In those areas where the original structures of the Lewisian gneiss have not 

 been effaced by later mechanical stresses it is possible to trace knots, bands, and 

 lenticles of unfoliated, ultrabasic, and basic rocks to note the imperfect separation 

 of the ferro-magnesian from the quartzo-felspathic constituents, to observe the 

 gradual development of mineral banding and the net-like ramification of acid 

 veins in the massive gneisses. Many of these rocks cannot be appropriately 

 described as gneiss. Indeed, Mr. Teall has called attention to the close analogy 

 between these structures and those of plutonic masses of younger date. 



In the Report on Survey Work in the North-west Highlands, published in 

 1888, the parallel banding, or first foliation, as it was then termed, of these original 

 gneisses was ascribed to mechanical movement.^ But the paper on ' Banded 

 Structure of Tertiary Gabbros in Skye,' by Sir A. Geikie and Mr. Teall,^ throws 

 fresh light on this question. In that region the gabbro displays the alternation of 

 acid and basic folia, the crumpling and folding of the bands like the massive 

 gneisses of the Lewisian complex. Obviously in the Skye gabbro the structures 

 cannot be due to subsequent earth movements and deformation. The authors 

 maintain that they are original structures of the molten magma, and, consequently, 

 that much of the mineral banding of the Lewisian gneisses, as distinguished from 

 foliation, may be due to the conditions under which the igneous magma was 

 erupted and consolidated. Whatever theory be adopted to explain the original 

 mineral banding of the Lewisian gneisses, it is certain that they possessed this 

 banding, and were thrown into gentle folds before the uprise of the later intrusive 

 dykes. 



The crystalline schists that have affinities with rocks of sedimentary 

 origin occupy limited areas north of Loch Maree and near Gairloch. The pro- 

 minent members of this series are quartz schists, mica schists, graphitic schists, 



' Report on the Recent Work of the Geological Survey in the North-west High- 

 lands of Scotland based on the Field-notes and Maps of Messrs. B. N. Peach, 

 J. Home, W. Gunn, C. T. Clough, L. W. Hinxman, and H. M. Cadell, Quart. Journ. 

 Gaol. Soc, vol. xliv. p. 387 ; and Annual Report of the Geological Survei/ for 1894, 

 p. 280, and 1895, p. 17. 



' Annual Report of the Geological Survey for 1894, p. 280. 



» Quart. Journ. Oeol. Soc., vol. xliv. p. 400. 



* Jbid., vol. 1. p. 645. 



