98 THE VOYAGE OF H.M.S. CHALLENGER. 



tion of the mass, as shown on the shore, exhibits an enormous accumulation of stones, 

 and the river flows out from beneath an archway of piled-up blocks. As we have 

 said, the interstices of the heaps are carpeted with moss. 



The inhabitants view these " stone-rivers " as one of the marvels of their island, 

 and explain their formation by the most improbable hypotheses. Darwin seems to 

 have accounted for them by great earthquakes in the region, but does not consider 

 this a sufficient interpretation. Thomson suggests another explanation. The blocks 

 of quartzite filling the valleys may come from the shelves of rock which appear 

 on the surrounding hills (Darwin remarked that they might come laterally from the 

 nearer slopes as well), and these piled-up blocks certainly show great lithological 

 analogies with the higher beds. The difficulty of the problem comes in when we try 

 to explain how the stones should descend in a close mass along a valley, the slope of 

 which, according to Darwin, is not steep enough to hinder the passage of a coach. 

 The slope in fact does not exceed 6° or 8° ; usually it is only 2° or 3°, and it is never 

 great enough to allow the stones to roll, or even slide, down. According to Thomson, 

 the quartzite shelves of the hill-tops do not all resist disintegration equally, the softer 

 parts weather into sand, and the harder, being left without support, break off into 

 irregular blocks. This explanation is equally applicable to the crystalline rocks, the 

 presence of which we are about to show amongst the ddbris. When the fragments 

 break off vegetation rapidly covers them up, and many of the little mossy heaps are 

 only stones covered by a thin layer of vegetation. Once enclosed in this mass they are, 

 as it were, pushed over the slope. We may mention, amongst other causes that act 

 as well as gravitation, the expansion and contraction of the moss as it takes up more 

 or less water. The dilatation of the moss moves the blocks, and the superficial layer of 

 stones is in some degree drawn towards the declivity. Rain washes off the sandy 

 debris ; this erosion prepares the way for the larger blocks, while on the other hand the 

 adjacent vegetable matter decomposes and is washed away. It is to the slow removal 

 of vegetable and mineral matter, and to the movement of the superficial layers — of 

 which Thomson gave numerous examples observed by him in Scotland — that he 

 attributes the accumulation of stones in the valleys. 



Neither Thomson nor Darwin have called in ice-action as a means of trans- 

 port, although it has been alleged that the Falkland Islands were covered by 

 glaciers at an epoch not very far removed from our own. No certain proofs of 

 glaciation are to be seen in the islands, and the stones of these streams bear 

 no marks of glacial striae. Only a detailed study of local conditions would enable 

 us to say whether Thomson's theory gives an adequate explanation of all the 

 facts. None the less is it true that this theory seems preferable by its simplicity 

 to that which Darwin demanded, when he wrote, forty years ago, on the subject of 



