2o6 



HARDWICKE'S SCIENCE-GOSSIP. 



l^hysical aspect of West Sutherland seems truly 

 extraordinary. Here is developed in extensive areas 

 the Archaean Gneiss formation, the patriarch of 

 British rocks, the uppermost layer of the under crust 

 of the earth, " the very core, so to speak, the central 

 part of the known stratified rocks of the earth." The 

 type of scenery peculiar to this formation is very 

 singular, and well worthy of portrayal. Perhaps the 

 most remarkable feature consists of a vast, monotonous 

 congregation of mounds, knolls, hummocks, and 

 bosses of rock with hollows between ; the whole 

 rising to a pretty uniform level. Now there is seen 

 a quasi-semicircular serrated slope, now a sharp 

 pinnacle, or a steep precipice ; a vertical acclivity, 

 rocky and ridgy, studded and flecked with cold, grey, 

 hoary rocks, verdureless everywhere, save only for an 

 occasional upshot of stunted, olive-green, mossy 

 blades of heather and grass, ora few trees straggling near 

 the dreary lochs. Viewed from an elevation, there is 

 seen sometimes the likeness of ridges rising up like 

 storm-vexed billows, and then suddenly petrified and 

 made to cease, rendered cold and lifeless for evermore ; 

 bright as may be occasionally, but then again, further 

 away, deep purplish and dun. 



Gneiss is a crystalline-granular compound of 

 quartz, felspar, and mica, arranged with foliated 

 texture, i.e. the original magma has been so 

 chemically and physically altered, that these resulting 

 minerals are now, to a certain extent, arranged in 

 parallel layers, and not promiscuously. The quartz 

 and felspar crystals exhibit a curved, wavy, or lobate 

 outline (in granite it is mainly rectilinear). The 

 general strike of the formation in Sutherland is 

 N.W. and S.E., and there is great uniformity in 

 lithological characters. The principal feature is a 

 "coarse hornblende gneiss, with distinct zones of 

 gray and pink granitoid gneiss wherein the mica is 

 more abundant than the hornblende." This matrix 

 or foundation rock is extensively intersected by veins 

 of pink or white pegmatite (a very coarse sort of 

 granite), and of hornblende-rock and hornblende- 

 schist, and by dykes of pink granite (as at Cape 

 Wrath, &c.), while small kernels of cleavable horn- 

 blende and of actinolite (a green variety of horn- 

 blende) occasionally occur. The principal minerals 

 associated therewith are garnets, anthophyllite, chert, 

 moss-agate, steatite, ripidolite, agalmatoHte, titan- 

 iferous iron, &c. Numerous folds occur in these 

 rocks ; but, according to the best authorities, these 

 rocks do not present any traces of a sedimentary or 

 aqueous origin. They contain no fossils, are highly 

 crystalline, and their planes of foliation are evidently 

 not vestiges of planes of deposition, but are really 

 planes of shearage and cleavage ; for in many places 

 the obvious effects of a shearing crush (such as " tail- 

 less " felspar, undulating mica, and uniform, lineally- 

 arranged quartz) are abundantly manifest. On the 

 other hand, that these rocks were not originally 

 sedimentary and then subsequently metamorphosed. 



is just about as palpably evident. It is tolerably 

 certain, indeed, that their origin consisted of the 

 same sort of molten magma which produces true or 

 igneous granite, the difference in the two cases being 

 that in lieu of being formed quietly, and at great 

 plutonic depths, the constituent materials of the 

 gneiss have been violently erupted from the interior 

 in a state of solution in water of an exceedingly high 

 temperature, and have crystallised therefrom, under 

 circumstances of vast contemporaneous disturbance 

 and of horizontal and vertical movement. There is 

 every indication that this part of Scotland and the 

 outer Hebrides consisted in former times of a molten 

 magma of rock, wavy and mobile, and bearing on its 

 fiery billows colossal piles of grit and sand, which 

 ultimately consolidated on the higher bosses and 

 platforms of the denser and more highly compounded 

 material below. 



Reared for the most part on these high uplifted 

 platforms or plateaux of arch^an gneiss are seen 

 colossal piles and domes of a bright red gritty or 

 sandstone rock, representative of the nietamorphic 

 Cambrian formation. This is stratified in tiers and 

 courses wonderfully distinct, uniform, and horizontal. 

 This formation, as revealed in various localities in 

 the county, consists of series of breccias, con- 

 glomerates, grits, and red sandstones, which are 

 markedly unconformable to their basement rock, and 

 are quite unfossiliferous. The grits, which are the 

 predominant form, are uncommonly similar in 

 appearance to a bit of well-burnt fireclay, and 

 suggest at once the idea of an igneous origin. No, 

 quoth the Scotch geologist, they certainly have been 

 originally sedimentary (i.e., we suppose they have 

 been mechanicallydeposited in " the cold," as chemists 

 say), and have subsequently been subjected to the 

 metamorphic agencies of heat, pressure, and water. 

 And again, inasmuch as the eminently picturesque 

 mountain of Suilven (the "Sugarloaf" of sailors) is 

 formed of the same Cambrian rock as are the 

 mountains Canisp, Quinag, Coulmore, &c., and as 

 great gaps and hollows, some thousands of feet deep 

 and wide, now subsist between these, it has been 

 urged or concluded that vast piles of Cambrian grits 

 and sandstones once covered the whole interspaces 

 now breached and hollowed out by aerial denudation. 

 But it so happens that the underlying gneiss is seen 

 to run high up on tlie flanks of these hills ; and as 

 this rock has not evidently been very much denuded, 

 it might be conceded that the Cambrian strata 

 perched and exposed atop of these more protruding 

 bosses of gneiss would be all the more likely to yield 

 to abrading agencies there than elsewhere. If it be 

 admitted that the grits and sandstones have been so 

 very seriously attrited, then it would not be too much 

 to expect that the gneiss, although considerably 

 tougher, would be likewise more or less worn away — 

 an anticipation hardly compatible with the facts 

 gathered from an orographical and lithographical 



