5IO 



NA rURE 



[September 19, 1901 



the principles of geology for his students. At that meeting, 

 also, the late Duke of Argyll, whose interest in geological ques- 

 tions never faded, gave an address "On the Physical Structure 

 of the Highlands in connection with their Geological History."' 

 The return of the Association to the second city of the empire, 

 which since 1876 has undergone remarkable development, due 

 in no small measure to the mineral wealth of the surrounding 

 district, suggests the question, Has Scottish geology made im- 

 portant advances during this interval of time ? Have we now 

 more definite knowledge of the geological systems represented 

 in Scotland, of their structural relations, of the principles of 

 mountain-building, of the zonal distribution of organic remains, 

 of the volcanic, plutonic, and metamorphic rocks so largely 

 developed within its borders ? It is true that many problems 

 still await solution, but anyone acquainted with the history of 

 geological research must answer these questions without hesita- 

 tion in the aftirniative. In the three great divisions of geological 

 investigation— in stratigraphical geology, in pala-ontology, in 

 petrology — the progress has indeed been remarkable. The 

 details of these researches are doubtless familiar to many who 

 have taken an active share in the work, but it may serve a 

 useful purpose, and perhaps be helpful as a landmark to give 

 now an outline of some of the permanent advances in the solid 

 geology of Scotland during the last quarter of a century. 



The belt of Archrean gneisses and schists, which may be said 

 to form the foundation stones of Scotland, have been mapped in 

 great detail by the Geological Survey since 1SS3 along the 

 western part of the mainland in the counties of Sutherland and 

 Ross. In that region they occupy a well-defined position, being 

 demonstrably older than the great sedimentary formation of 

 Torridon Sandstone and overlying Cambrian strata. The 

 mapping of this belt by the survey staff and the detailed study of 

 the rocks both in the field and with the microscope by Mr. Teall 

 have revealed the coniple.xity of the structural relations of these 

 crystalline masses, and have likewise thrown considerable light 

 on their history. These researches indicate that, in the North- 

 west Highlands, the Lewisian(Archrean) gneiss may be resolved 

 into (i) a fundamental complex, composed mainly of gneisses 

 that have affinities with plutonic igneous products, and to a 

 limited extent of crystalline schists which may without doubt be 

 regarded as of sedimentary 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 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, com- 

 prehensive, 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 

 ("Annual Report of the Geological Survey for 1894," p. 2S0). 

 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 Lax- 

 ford ; the second, from near Scourie to beyond Lochinver, and 

 the third from Gruinard Bay to the island of Raasay. 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 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, 



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. Sec, vol. xliv. p. 387 ; and "Annual Report of 

 the Geological Survey for 1894," p. z8o, and 1S95, p. 17. 



NO. 1664, VOL. 64] 



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 High- 

 lands, published in iSSS, the parallel banding, or first foliation, 

 as it was then termed, of these original gneisses was ascribed 

 to mechanical movement {Quart . Joitrn. Geoi. Sot., vol. xliv. 

 p. 400). But the paper on " Banded Structure of Tertiary 

 Gabbros in Skye," by Sir A. Geikie and Mr. Teall (Hid., 

 vol. 1. p. 645), 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 latter 

 intrusive dykes. 



The crystalline schists that have affinities with rocks of sedi- 

 mentary origin occupy limited areas north of Loch Maree and 

 near Gairloch. The prominent members of this series are quartz 

 schists, mica schists, graphitic schists, limestones and dolomites 

 with tremolite, garnet and epidote (" Annual Report of the Geo- 

 logical Survey for 1895," p. 17). They are there associated 

 with a massive sill of epidiorite and hornblende schist. The 

 relations which these altered sediments bear to the gneisses that 

 have affinities with plutonic igneous products have not been 

 satisfactorily determined. But the detailed mapping has proved 

 that north of Loch Maree they rest on a platform of Lewisian 

 gneiss, and are visibly overlain by gneiss with basic dykes (Meall 

 Riabhach), and that both the gneiss complex and altered sedi- 

 ments have been afi'ected by a common system of folds. In the 

 field, bands of mylonised rock have been traced near the base 

 of the overlying cake of gneiss, and the microscopic examination 

 of the latter bj' Mr. Teall has revealed cataclastic structures due 

 to dynamic movement. It is obvious, therefore, that, whatever 

 may have been the original relations of the altered sediments to 

 the gneiss complex, these have been obscured by subsequent 

 earth-stresses. 



The great series of later igneous rocks which pierce the funda- 

 mental complex in the form of dykes and sills is one of the 

 remarkable features in the history of the Lewisian gneiss. In 

 1895 Mr. Teall advanced a classification of them (ibid., p. 18), 

 but his recent researches show that they are of a much more 

 varied character. For our present purpose we may omit the 

 dykes of peculiar composition and refer to the dominant types. 

 These comprise: (i) ultrabasic rocks (peridotite), (2) basic 

 (dolerite and epidiorite), and (3) acid (granite and pegmatite). 

 The evidence in the field points to the conclusion that the ultra- 

 basic rocks cut the basic, and that the granite dykes were in- 

 truded into the gneisses after the eruption of the basis dykes. 

 The greater number of these dykes consists of basic materials. 

 It is important to note that the basic rocks best preserve their 

 norinal dyke-like features in the central tract between Scourie 

 and Lochinver, where they traverse the pyroxene gneisses. But 

 southwards and northwards of that tract, in districts where they 

 have been subjected to great dynamic movement, they appear as 

 bands of hornblende-schist, which are difficult to separate from 

 the fundamental complex. The acid intrusions are largely 

 developed in the northern tract between Laxford and Durness ; 

 indeed, at certain localities in that region the massive and foliated 

 granite and pegmatite are as conspicuous as the biotite gneisses 

 and hornblende gneisses with which they are associated. 



After the eruption of the various intru-sive dykes the whole 

 area was subjected to enormous terrestrial stresses vvhich pro- 

 foundly affected the fundamental complex and the dykes which 

 traverse it. These lines of movement traverse the Lewisian 

 plateau in various directions, producing planes of disruption, 

 molecular rearrangement of the minerals and the development 

 of foliation. It seems to be a general law that the new planes 



