REPORT ON THE PETROLOGY OF OCEANIC ISLANDS. 97 



VII.— EOCKS OF THE FALKLAND ISLANDS. 



A. Rocks of the "Fivers of Stones." 



The Falkland Islands are connected by their geological character with the American 

 Continent, thus presenting a marked contrast to the oceanic islands of the Atlantic, 

 most of which are formed exclusively of volcanic rocks. The Falklands, on the con- 

 trary, are made up of sedimentary strata — schists, sandstones, and quartzite of Silurian 

 and Devonian age — and archasan rocks. We shall here limit ourselves to the con- 

 sideration of those remarkable " stone rivers " which form one of the most interesting 

 features of these islands, and we propose to describe the lithological nature of some of 

 the rocks of these " streams." Both Darwin and Wyville Thomson examined them with 

 close attention, and described them. Combining their descriptions, 1 we may obtain 

 an idea of the origin of these stony accumulations. 



At the east end of the principal island in the Falkland group the valleys present 

 a most striking appearance, being filled with masses of pale grey rocks, which glitter 

 in the sun, and form tracks of from a few hundred to more than a thousand metres in 

 breadth. From a little distance the effect is that of a gigantic glacier, descending from 

 the neighbouring heights and gradually increasing in volume, as if it were fed by lateral 

 streams up to the point where the main " river " reaches the coast. The stones, which 

 vary in size from 30 centimetres to 7 metres, are not piled up irregularly, but extend 

 in great level beds varying from 100 to 1900 metres in width. Thomson showed 

 that the width of the stream is always in relation to that of the shelves of rock 

 which crown the hills. Deposits of peat are constantly encroaching on the flows, and 

 even form islands, when the fragments are near enough to afford a basis. Immense 

 masses of rock on the hills seem to have been stopped in their course, and frag- 

 ments, bending over like arches, are piled upon each other like the ruins of an ancient 

 cathedral. 



All those who have visited the Falklands agree in saying that the stones in ques- 

 tion are not water-borne, but are angular, like the fragments of a breccia, and piled 

 up irregularly one above another. They are not decomposed, except to such an extent 

 as might be due to ordinary atmospheric agencies ; the angles are generally worn, with 

 a shining, slightly-polished surface. A thin coating of whitish lichen covers the stones, 

 giving them quite the appearance of ice from a little distance. The thickness of the 

 layer of stones is not easily determined, but the sound of running water may be 

 heard evidently a few feet beneath the surface. At the mouth of the valley the sec- 



1 Darwin, Voyage of a Naturalist ; Thomson, The Atlantic, vol. ii., p. 216. See also Karr. Chall. Exp., vol. i., p. 89-'. 



(PHTS. CHEM. CHALL. EXP.— PART VII. — 1889.) 13 



