July 25, 1889] 



NATURE 



301 



•with the north-west dykes, the second trends east and 

 west, and the third runs north-east and south-west, or 

 north and south. So far as yet discovered, no lava of 

 any kind welled upwards into these fissures. They are 

 ruptures, but not dykes. They were accompanied, how- 

 ever, by the manifestation of another form of terrestrial 

 energy, the geological efficacy of which has only recently 

 been recognized. The lines of vertical fracture became 

 also lines of horizontal or oblique movement during the 

 vast strain of terrestrial contraction. One side was driven 

 past the other side, and with such irresistible force that 

 the rocks for some distance on either side were dragged 

 into the line of movement, crushed down, and forced to 

 assume a new crystalline arrangement of their materials. 

 The basalt dykes, reduced sometimes from a width of 

 50 or 60 yards to only 4 feet or less, were changed into 

 diorites, and where the shearing was greatest, into horn- 

 blende-schists. The gneiss, in like manner, was thrown 

 into sharp folds, and had a newer foliation developed in 

 it parallel with the new planes of movement. 



In the fourth place, during the prolonged succession of 

 changes which I have thus briefly summarized, there 

 must have been in progress a continuous denudation of 

 the surface of the Archaean land in the north-west of 

 Europe. Doubtless, each of the subterranean disturb- 

 ances more or less affected the surface. The land was 

 by degrees ridged up above the sea, and its height and 

 breadth were probably from time to time increased by 

 local uplifts accompanying the disturbances. But as soon 

 as the land appeared, it began ■ to be attacked by the 

 waves, the air, rain, and running water. Terrestrial con- 

 vulsions were intermittent, but superficial waste continued 

 uninterrupted. Whatever may have been the character 

 of its topography, the first formed land, as soon as it rose, 

 became a prey to the denuding forces, and had its 

 original surface gradually stripped off. We have no 

 means of telling how great a thickness of material was 

 in this manner removed from the land before the time 

 of the next geological period, nor for how vast a time this 

 slow process of denudation went on. All that we can 

 now discover is a series of detached fragments of the 

 surface of this primaeval Europe, which have been pre- 

 served by being buried under the pile of material formed 

 out of the waste of the Archsan rocks. From these frag- 

 ments we learn that the rocks had been enormously 

 denuded so as to lay bare to the surface some of their 

 deep-seated parts, the land shaped out of them having 

 been carved into dome-shaped hills and basin-like 

 hollows, not very different from those which are so 

 characteristic of.the Archaean tracts to-day. 



II. — The Catnbrian Period. 

 We now reach the base of the stratified formations of 

 the British Isles, and enter upon a series of records which 

 deal not with subterranean but with superficial changes, 

 and in which the earliest geographical conditions of our 

 area are more or less fully chronicled. These records 

 consist of a pile of dull-red sandstones, conglomerates, 

 and breccias, with grey, green, and black mudstones, 

 marls, and shales, attaining a maximum thickness of per- 

 haps 10,000 feet. This great accumulation, chiefly of 

 coarse sediment, was derived from the waste of the 

 Archaean land. The pebbles in its conglomerates are 

 fragments of that land, and enable us to form some 

 conjecture as to the nature of the materials that 

 composed its surface. An examination of these pebbles 

 brings to light the important fact that besides the detritus 

 of the gneiss and other Archaean rocks which can now be 

 seen in situ, the conglomerates are made up of materials 

 derived from some still older sedimentary formations which 

 have entirely disappeared from our area. These included 

 such rocks as quartzite, greywacke, shale, and limestone, 

 besides abundant pieces from the lavas, which I have 

 already referred to as having probably been erupted to 



the surface in pre-Cambrian time. The destruction of 

 these intervening deposits, and the chance discovery that 

 they once existed because fragments of them have been 

 found in later conglomerates, serve to impress upon 

 us the imperfection of the geological record, and the 

 vastness of the intervals "of time which may sometimes 

 separate two successive groups of rock. 



The thick mass of red sandstone and conglomerate 

 which rests directly on the Archaean gneiss forms some 

 of the most singular scenery in the north-west of Scot- 

 land. Owing to vast denudation, which began before 

 the next group of strata was deposited, it has been worn 

 down into isolated mountains, which rise like a chain of 

 colossal pyramids along the western shores of Sutherland 

 and Ross. The almost level lines of stratification give 

 to these eminences a look of architectural symmetry, in 

 striking contrast with the more tumultuous aspect of the 

 other rocks of the region, while their red tone of colour 

 marks them out boldly from the wastes of grey gneiss 

 below and the crags of white quartzite beyond. From 

 the far northern cliffs of Sutherland these massive red 

 sandstones can be followed almost continuously to the 

 southern headlands of Skye. They reappear in great 

 fofce in the Island of Rum, beyond which they are not 

 certainly traceable. A group of highly altered grits and 

 schists, seen under the great basaltic plateau of Gribun, 

 on the west side of the Island of Mull, may mark their 

 extreme southerly limits.^ The red sandstones certainly 

 do not come so far south as lona, and not a trace of 

 them has been met with in Ireland. They extend west- 

 wards across the Minch, for a small portion of them 

 skirts the eastern shore of the Long Island. How far 

 they may have stretched eastward cannot now be deter- 

 mined, for their limits in that direction have been 

 obscured or effaced by the extraordinary series of 

 gigantic earth-movements to be afterwards referred to. 

 There can be little doubt, however, that they did not 

 reach the district east of the line of the Great Glen, though 

 they not improbably lay in thick mass over much of the 

 country to the west of that valley. 



We cannot now trace the original limits of these red 

 rocks, yet we can hardly doubt that they never covered 

 an area at all comparable in extent to that of the rocks 

 below and above them. They appear, indeed, to have 

 been accumulated in one or more basins, shut off from 

 free communication with the open sea, where the deposition 

 of ferruginous precipitates among the ordinary mechanical 

 sediment could go on during the deposition of many 

 thousand feet of rock. Such conditions of sedimentation 

 were not very favourable to the existence of life in the 

 waters of these inclosed basins. Nevertheless, that the 

 waters were not entirely lifeless is shown by the discovery 

 of organic remains on two widely separated horizons 

 among the sandstones. These remains occur in grey 

 and dark shales, the colour and composition of which 

 suggest a temporary influx of water from without, and 

 the cessation for a time of the deposition of the iron- 

 oxide. At the lower horizon the fossils consist of cal- 

 careous rods, the organic grade of which is still in dispute ; 

 at the higher they include some doubtful impressions and 

 the casts of worms. The fossiliferous bands are to be 

 more thoroughly searched this summer, and it is hoped 

 that something more determinable may be obtained from 

 them. 



Nevertheless, indistinct though these relics undoubtedly 

 are, they may claim the interest which arises from their 

 being at present the very oldest traces of organized 

 existence yet found within our islands. Murchison 

 classed the red sandstones of Western Sutherland and 

 Ross as " Cambrian," inasmuch as he found them to 



' My attention was called to these rocks by the Duke of Argyll, who 

 himself suggested their possible Cambrian age. I visited them this spring, 

 and found them to be greatly metamorphosed. They do not appear in lona, 

 where the base of the sedimentary series is found resting on the Archaean 

 gneiss. 



