427 
Tierra del Fuego, and Sirait of Magellan.—Yhe eastern portion of 
Tierra del Fuego is formed of large outliers of the Patagonian form- 
ation, fringed “by deposits of more recent origin. These lower 
plains, varying in height from 100 to 250 feet, have been elevated 
within the post-pleiocene period ; and they consist of finely grained 
argillaceous sandstone arranged in thin horizontal or inclined lamin, 
and often associated with curved layers of gravel. On the eastern 
borders of the Straits of Magellan, and at "Elizabeth Island, Cape 
Negro, Nuestra Sefora 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 thining 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 isletsioff 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 
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