Nov. 13, 1S84] 



NA TURE 



3* 



than ninety miles. The task of unravelling the geological 

 structure of these southern regions will be much facilitated 

 by the remarkable persistence of the Sutherland Silurian 

 zones, some of which, with their characteristic features 

 and fossils, are as well marked above Loch Carron as 

 they are at Loch Eriboll. 



In south-western Ross-shire the platform on which the 

 Silurian rocks rest is a thick mass of Cambrian red sand- 

 stone. In the great upthrow, it is this sandstone platform 

 which has there been pushed over the limestones and 

 quartzites. On the west side of Loch Keeshorn, the red 

 sandstones, in their normal unaltered form, rise up into 

 the colossal pyramids of Applecross ; but on the east 

 side, where, at a distance of little more than a mile, they 

 overlie the limestones, they bear so indurated an aspect 

 that they have naturally been classed with the 

 quartzose members of the Silurian series. Traced east- 

 wards they present increasing evidence of intense shear- 

 ing ; fluxion-structure makes its appearance in them, with 

 a development of mica along the divisional planes, until 

 they pass into frilled micaceous schist, in which, how- 

 ever, the original clastic grains are still recognisable. 

 They finally shade upwards into green schists and fine 

 gneiss which merge into coarse gneiss with pegmatite. 

 The short space within which ordinary red feldspathic sand- 

 stone and arkose acquire the characters of true schists is 

 a point of some importance in regard to the change from 

 the unaltered Silurian strata of the Southern Uplands into 

 the metamorphic condition of the Highland phyllites, 

 grits, &c. 



Obviously the question of chief importance in connec- 

 tion with the structure now ascertained to characterise 

 the North-West Highlands relates to metamorphism. That 

 there is no longer any evidence of a regular conformable 

 passage from fossiliferous Silurian quartzites, shales, and 

 limestones upwards into crystalline schists, which were 

 supposed to be metamorphosed Silurian sediments, must 

 be frankly admitted. But in exchange for this abandoned 

 belief, wc are presented with startling new evidence of 

 1 metamorphism on a colossal scale, and are 



I some way into the secret of the processes 

 whereby it has been produced. 



From the remarkably constant relation between the 



dip of the Silurian strata and the inclination of their 



faults, no matter in, ous positions the 



uires may have been thrown, it is tolerably clear 



thai the disl is took place before the strata had 



riously disturbed. The persistent parallelism of 

 the faults and of the prevailing north-easterly strike of 

 the rocks indicates that the faulting and till 

 of one continuous process. The same dominant north- 

 easterly strike extends across the whole Highlands, and 

 ■ 1 ithern Scotland and 

 the North of England. There is reason to regard it in 

 all these regions as probably due to one great series of 

 1 ments. These must have occurred some 

 ween an early part of the Silurian period and 



1 of the Old Red Sandstone period represented 



and conglomerates of the Highlands. In 



itral and Eastern Highlands the slate . phyl- 

 ,-,es which, along the 

 1, are scarcely more altered than their 

 Jents among the Silurian rocks of the 

 Southern Uplands, have been greatly pli • 

 assumed a more or less crystalline structure. But when 

 there lay to the north- 

 west a solid ridge of Archaean gneiss and Cambrian 

 sandstone whi h otic. istance to the plication. 



The thrust from the eastward against this ridge must 

 have been of the most gigantic kind, for huge slices, 

 hundreds of feet in thickness, were shorn off from the 

 quartzites, limestones, red sandstones, and gn< 

 were | ushed for miles to the westward. During this 

 process, all the rocks driven forward by it had their 



original structure more or less completely effaced. New 

 planes, generally parallel with the surfaces of movement, 

 were developed in them, and along these new planes a 

 rearrangement and recrystallisation of mineral con- 

 stituents took place, resulting in the production of crystal- 

 line schists. This metamorphism certainly occurred after 

 early Silurian times, for Cambrian and Lower Silurian 

 strata, as well as Archaean rocks, have been involved in it. 

 It is obvious that into the problems of Highland 

 geology, always admittedly obscure, a fresh element of 

 difficulty is introduced. At the same time the aid fur- 

 nished by a minute study of the Sutherland sections is so 

 great that we may hope to attack these problems with 

 more success than has hitherto seemed probable. The 

 work, too, is not of a kind to be attempted in a few hasty 

 scampers over the ground. It will require patient detailed 

 mapping. But when the great base-lines have once been 

 accurately traced, the difficulties will doubtless begin to 

 diminish, and, like the pieces of a puzzle, the various 

 segments of the Highlands will then be found to range 

 themselves in their proper places. Arch. Geikie 



Report mi the Geology of the North- West of Sutherland 



Ix the north-west of Sutherland the most ancient 

 rocks belong to the Archaean series, and present a 

 great uniformity in lithological characters. They consist 

 mainly of coarse hornblendic gneiss, with distinct zones of 

 graj and pink granitoid gneiss, in which the mica is more 

 : mt than the hornblende. Lenticular veins and 

 of hornblende-rock and hornblende-schist, some at 

 least of which are evidently intrusive, occur in the gneiss, 

 while the presence of small kernels of cleavable horn- 

 blende and actinolite forms another characteristic feature 

 of the series. Veins of pink or white pegmatite abound, 

 sometimes parallel with the foliation of the gneiss and 

 sometimes traversing it in all directions. These, how- 

 ever, are distinct from dykes of pink granite, which also 

 intersect the gneiss and coarse pegmatites, and are them- 

 selves crossed by later pegmatite-veins. Here and there, 

 indeed, the branches of a pegmatite-vein can be seen to 

 return upon themselves and traverse the main trunk from 

 which they start. Where the Archaean rocks have been 

 recently stripped of their former cover of Silurian quartzite, 

 bands of green epidotic gneiss appear among them, and a 

 soft green mineral with a greasy lustre (agalmatolite ?) is 

 there characteristic of the superficial parts of the pegma- 

 tite-veins. 



The highly crystalline Archaean rocks arc overlain un- 

 conformably by a succession of conglomerates, grits, and 

 sandstones, regarded by Murchison as the equivalents of 

 the Cambrian system of Wales. In the course of the 

 work of the Geological Survey in the present region they 

 have been divided into certain zones, which, though they 

 need not be stated here, as they have no bearing on the 

 main question to which this paper is devoted, may prove 

 to be of considerable importance in unravelling the geo- 

 logical structure of the districts further south. 



Between the Cambrian sandstones and the overlying 

 quartzites at the base of the Silurian series there is a 

 complete discordance. To the west of the Kyle of 

 Durness, for example, the Cambrian sandstones dip to 

 the north-west, while the overlying quartzites dip to the 

 south of east. Moreover, as the observer passes east- 

 wards to the shores of the Kyle, the Cambrian sandstones 

 arc bed after bed transgressed by the quartzites, which 

 eventually rest directly on the Archaean gneiss. The 

 n strata in the Durness area (a in Section) consist 

 ilcareous series at the top ; a middle series, com- 

 posed partly of calcareous and partly of arenaceous strata; 

 and an arenaceous series at the base. The various sub 

 ons of the strata arc given in descending order in 

 the subjoined tabular statement. 



