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15 feet in length, 11 in breadth, and 9 in height ; another, of a pen- 
tagonal form, 11 feet on each side, and at one part projected 16 feet 
above the sand, in which it was partly buried. 
At the extreme northern point of Chiloe, a headland 250 feet 
high is joined to Lacuy peninsula by a low neck of land; and from 
its composition, height and stratification, Mr. Darwin ascertained 
that it was once continuous with the opposite coast. The boulders 
were much more numerous on the isthmus and its sides at the 
height of 150 feet, than on any other part of the surrounding coun- 
try; and as the sea must have flowed over this isthmus in a channel, 
previous to the amount of elevation, ascertained to have taken place 
here within the post-pleiocene period, the position of these boulders 
proves, according to Mr. Darwin, even more clearly than the cases 
occurring in Tierra del Fuego, the evident relationship between 
their distribution and the lines of anciently existing sea-channels. 
In the southern half of Chiloe, and on one of the Chonos islands, the 
author discovered a deposit of hardened mud, including far trans- 
ported, angular and rounded fragments, and resembling the till of 
the Straits of Magellan. In a layer of loose sand at the base of the 
cliff in the latter locality, he noticed a quantity of comminuted ma- 
rine shells with a fresh aspect ; and at Chiloe he also observed, at a 
point where a mass of till passed into finely grained laminz, small 
fragments of a Cytherea. 
With respect to the age of the boulder formation of Chiloe, Mr. 
Darwin offers no precise remark, but he says that it probably occurs 
within the post-pleiocene period, because at a height of 350 feet on 
the peninsula of Lacuy, and therefore considerably above the level 
of this formation, a great bed of existing sea-shells was observed, 
and neither the boulder nor accompanying beds appear to have been 
of deep-water origin. Similar evidence was adduced respecting the 
age of the till of Tierra del Fuego. North of 41° 47’ 8. lat., Mr. 
Darwin did not observe on the Pacific side of South America either 
boulders or till; nor any north of the Straits of Magellan, on the 
shores of the Atlantic side; and he accounts for the absence of erra- 
tic blocks in the latter region by its great distance from the Cor- 
dillera...He is also strongly of opinion that till will be found to 
be limited to the latitudes in which true boulders occur. 
Glaciers, &c.—In the concluding part of his memoir, the author 
offers a few remarks on the glaciers of Tierra del Fuego, and on the 
transport of the boulders. He did not disembark on any glacier, 
but in the Beagle and Magdalen channels he passed within 2 miles 
of several. The mountains were covered with snow, and the gla- 
ciers formed many short arms, terminating at the beach in low per- 
pendicular cliffs of ice. Their surface, to a considerable height on 
the mountains, was perfectly clean and of a bright azure colour; and 
the former condition he ascribes to their shortness, to their not 
being flanked by overhanging precipices, and to their not being 
formed by the junction of two or more smaller streams. The de- 
scent of the glaciers, Mr. Darwin states, cannot be very slow, as vast 
masses continually break off with a great noise, and produce a tu- 
