Presidential Address to Geological Section. 453 



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 shown 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 classification of these 

 rocks, based mainly on their mineralogical composition and partly on 

 their structure, which has the great merit of being clear, comi:)rehensive, 

 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, hornblende, 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 districts, 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 of 

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

 ultrabasic rocks (pyroxenites and hornblendites) are specially developed, 

 while the granular hornblende rocks (hornblende gneiss proper) and the 

 biotite gneisses are characteristic of the northern and southern tracts. 

 These are the facts, whatever theory be 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 disj)lays 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 thoy 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, 



1 Report on the Recent Work of the Geological Survey in the North -West 

 Highlands 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. Geol. Soc, vol. xliv, p. 387 ; and Ann. Rep. Geol. Siu:v., 1894, p. 280, and 

 1895, p. 17. 



- Ana. Rep. Geol. Surv., 1894, p. 280. 



3 Quart. Journ. Geol. Soc, vol. xhv, p. 400. 



* Ibid., vol. L, p. 645. 



