538 Geological Society. — Mr. Darwin on the Erratic 



argillaceous sandstone arranged in thin horizontal or inclined laminae, 

 and often associated with curved layers of gravel. On the eastern 

 borders of the Straits of Magellan, and at Elizabeth Island, Cape 

 Negro, Nuestra Seflora de Gracia, all within the Straits, as well as 

 along the line of coast extending to Port Famine, the sandstone 

 passes into, or alternates with, great unstratified deposits, either of 

 an earthy nature and whitish colour, or of a hardened coarse-grained 

 mud of a dark colour, both containing angular and rounded fragments 

 as well as great boulders of sienite, greenstone, felspathic rocks, 

 clay-slate, hornblende-slate, and quartz. These are arranged without 

 the slightest indication of order, and are derived from mountains at 

 least 60 miles distant to the west or south-west. Sometimes the 

 mass is divided by beds of stratified shingle. North of Cape Vir- 

 gins, near the entrance of the Strait, it alternates with beds of argil- 

 laceous, horizontally laminated sandstone, often thinning out and 

 becoming curvilinear at each end. The inclosed fragments must, in 

 this case, have been transported at least 120 miles. Though Mr. 

 Darwin observed only two boulders imbedded in this deposit, yet 

 as he did not notice any scattered on the surface of the country, he 

 concludes that the boulders which occur in vast numbers on all the 

 beaches have generally been washed out of the cliffs : in St. Sebas- 

 tian's Bay, however, on the east coast of Tierra del Fuego, he found 

 many blocks in a protected position at the base of a naked cliff 200 

 feet high, entirely composed of thin strata of finely grained sand- 

 stone ; he therefore infers that, in this instance, they must have been 

 derived from a thin superficial deposit. From the form of the land 

 where these boulders occur, it is clear, Mr. Darwin states, that long 

 anterior to the present total amount of elevation, a wide channel 

 must have connected the middle of the Strait of Magellan with the 

 Atlantic ; and from the occurrence of boulders on the low neck of 

 land near Elizabeth Island, that at the same period a straight channel 

 must have existed between Otway Water and the eastern arm of the 

 Strait. As the present currents off Cape Horn set from the west, 

 Mr. Darwin says, it is probable that the ancient currents had a 

 similar direction, and this inference, he adds, is in accordance 

 with the fact, that the boulders and smaller fragments have been 

 transported from mountains to the west. Navarin Island, and 

 several adjacent islets off the extreme southern parts of Tierra 

 del Fuego, are fringed at about an equal height by an unstratified 

 boulder deposit, very similar to that of the Strait of Magellan ; and 

 in Beagle Channel, which separates Navarin Island from Tierra del 

 Fuego, it occasionally alternates regularly with layers of shingle. 

 This extensive deposit resembles, Mr. Darwin states, the "Till" of 

 Scotland, and the boulder formation of Northern Europe and the 

 East of England. The interstratification of regular beds, the occa- 

 sional appearance of stratification in the mass itself, the juxtaposition 

 of rounded and angular fragments of various sizes and kinds of rock 

 derived from distant mountains, and the frequent capping of gravel, 

 indicate some peculiar but similar origin in this deposit of the above 

 widely separated regions. Mr. Darwin follows Mr. Lyell in belie- 



