2,6o 



NATURE 



\Feb. 22, 1877 



in the ridges above. The blocks are angular, like the 

 fragments in a breccia, and they rest irregularly one upon 

 the other, supported in all positions by the angles and 

 edges of those beneath. 



They are not weathered to any extent, though the edges 

 and points are in most cases slightly rounded ; and the 

 surface, also perceptibly worn, but only by the action of 

 the atmosphere, is smooth and polished ; and a very thin, 

 extremely hard, white lichen, which spreads over nearly 

 the whole of them, gives them the effect of being covered 

 with a thin layer of ice. 



Far down below, under the stones, one can hear the 

 stream of water gurgling which occupies the axis of the 

 valley ; and here and there, where a space between the 

 blocks is unusually large and clear, a quivering reflection 

 is sent back from a stray sunbeam. 



At the mouth of the valley the section of the " stone 

 river " exposed by the sea is like that of a stone drain on 

 a huge scale, the stream running in a channel arched 

 over by loose stone blocks, or finding its way through the 

 spaces among them. There is scarcely any higher vege- 

 tation on the " stone river ; " the surface of every block 

 is slippery and clear, except where here and there a little 

 peaty soil has lodged in a cranny, and you find a few 

 trailing spikes of Nassauvia serpens, or a few heads of the 

 graceful drooping chrysanthemum-like Chabrcea suaveo- 

 lens. 



These " stone-rivers " are looked upon with great 

 wonder by the shifting population of the Falklands, and 

 they are shown to visitors with many strange specu- 

 lations as to their mode of formation. Their origin 

 seems, however, to be obvious and simple enough, and 

 on that account their study is all the more instructive ; 

 for they form an extreme case of a phenomenon which 

 is of wide occurrence, and whose consequences are, I 

 believe, very much ixnderrated. 



There can be no doubt that the blocks of quartzite in 

 the valleys are derived from the bands of quartzite in the 

 ridges above, for they correspond with them in every 

 respect ; the difficulty is to account for their flowing 

 down the valley, for the slope fjjom the ridge to the valley 

 is often not more than six to eight degrees, and the slope 

 of the valley itself only two or three, in either case much 

 too low to cause blocks of that form either to slide or to 

 roll down. 



The process appears to be this. The beds of quartzite 

 are of very different hardness ; some are soft, passing 

 into a crumbling sandstone ; while others are so hard as to 

 yield but little to ordinary weathering. The softer bands 

 are worn away in process of time, and the compact 

 quartzites are left as long projecting ridges along the 

 crests and flanks of the hill-ranges. When the process 

 of the disintegration of the softer layers has gone on for 

 some time the support of their adjacent beds is taken 

 away from the denuded quartzites, and they give way in 

 the direction of the joints, and the fragments fall over 

 upon the gentle slope of the hillside. The vegetation 

 soon covers the fallen fragments and usually near the 

 sloping outcross of the hard quartz, a slight inequality 

 only in the surface of the turf indicates that the loose 

 blocks are embedded beneath it. Once embedded in the 

 vegetable soil a number of causes tend to make the whole 

 soil-cap, heavy blocks included, creep down even the 

 least slope. 1 will only mention one or two of these. 

 There is constant contraction and expansion of the 

 spongy vegetable mass going on, as it is saturated witti 

 water or comparatively dry ; and while with the expan- 

 sion the blocks slip infinitesimally down, the subsequent 

 contraction cannot pull them up against their weight ; the 

 rain-water trickling down the slope is removing every 

 movable particle from before them ; the vegetable matter 

 on which they are immediately resting is undergoing a 

 perpetual process of interstitial decay and removal. In 

 this way the blocks are gradually borne down the slope 



in the soil-cap and piled in the valley below. The only 

 other question is how the soil is afterwards removed and 

 the blocks left bare. This, I have no doubt, is effected 

 by the stream in the valley altering its course from time 

 to time, and washing away the soil from beneath. 



This is a process which, in some of the great " stone- 

 rivers" in the Falkland Islands, must have taken an 

 enormous length of time. I fear that the extreme glacialists 

 will see in it a danger to the universal application of their 

 beloved theory to all cases of scratching and grooving. 

 I have known loo much of the action of ice to have the 

 slightest doubt of its power ; but I say that ice had 

 no hand whatever in the production of these grand 

 "moraines" in the Falkland Islands. 



In the West Highlands of Scotland, and in many other 

 parts of the world, I have often noticed that when 

 a hill of such a rock as clay-slate comes down with a 

 gentle slope, the outcrop of the vertical or highly-inclined 

 slates covered with a thick layer of vegetable soil or drift 

 containing imbedded blocks and boulders derived from 

 higher levels, the slates are frequently first slightly bent 

 downwards, then abruptly curved and broken, and fre- 

 quently the lines of the fragments of the fractured beds 

 of slate can be traced for a yard or two in the soil-cap, 

 gradually becoming parallel with its sur'ace, and passing 

 down in the direction of its line of descent. These move- 

 ments are probably extremely slow. I well remember 

 many years ago observing a case, somewhere in the west 

 of Scotland, where a stream had exposed a fine section 

 of the soil-cap with the lines of broken-down and crushed 

 slate-beds carried far down the slope. The whole effect 

 was so graphically one of vigorous and irresistible move- 

 ment that I examined carefully some cottages and old 

 trees in hope of fiinding some evidence of twisting or other 

 irregular dislocation, but there appeared to be none such. 

 The movement, if it were sufficiently rapid to make a 

 sign during the life-time of a cottage or a tree, evidently 

 pervaded the whole mass uniformly. 



It seems to me almost self-evident that wherever there 

 is a slope, be it ever so gentle, the soil-cap must be in 

 motion, be the motion ever so slow ; and that it is 

 dragging over the surface of the rock beneath the blocks 

 and boulders which may be embedded in it, and fre- 

 quently piling these in moraine-like masses, where the 

 progress of the earth-glacier is partially arrested, as at 

 the contracted mouth of a valley, when the water perco- 

 lating through among them in time removes the inter- 

 vening soil. As the avalanche is the catastrophe of ice- 

 movement, so the land-slip is the catastrophe of the 

 movement of the soil-cap. 



As I have already said, I should be the last to under- 

 value the action of ice, or to doubt the abundant 

 evidences of glacial action ; but of this I feel convinced, 

 that too little attention has been hitherto given to this 

 parallel series of phenomena, which in many cases it will 

 be found very difficult to discriminate ; and that these 

 phenomena must be carefully distinguished and elimi- 

 nated before we can fully accept the grooving of rocks 

 and the accumulation of moraines as complete evidence 

 of a former existence of glacial conditions. 



C. Wyville Thomson 



ON THE INFLUENCE OF GEOLOGICAL 

 CHANGES ON THE EARTH'S AXIS OF 

 ROTATION^ 



THE subject of the fixity or mobility of the earth's axis 

 of rotation in that body, and the possibility of 

 variations in the obliquity of the ecliptic has of late been 

 attracting much attention. Sir W. Thomson referred 

 shortly to it in his address at Glasgow last September, 



1 An account of a paper by G. H. Darwin, M A., read before the Royal 

 Society on November 23, 1876. 



