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long on the top of the mount,-un. Our descent was not quite
so laborious as our ascent; for the weight of the body forced
a pass.age', and .all the slips and falls were in the right direction.
C,apt,ain King has given a sketch of the geology of Tierra
del Fuego, to which I have little to add. A gxeat formation
of clay-slate, rarely containing organic remains, but
sometimes presenting casts of a kind of ammonite, is fronted
on the east side by pl.ains belonging probably to two tertiary
epochs. On the west coast, a prolongation of the grand
crevice of the Andes, from which so much heat has escaped
from the interior of the globe, has metamorphosed the slate.
There is, however, a double line, the structure of which I do
not quite comprehend. The interior one consists of granite
and mica slate; the exterior one (perhaps more modern), of
greenstone, porphyritic and other curious trappean rocks.
Almost every one at first thinks that this country owes its
gi-and name of “ the Land of Fire,” to the number of its
volcanoes. Such, however, is not the case: I did not see
even a pebble of any volcanic rock, except in Wollaston
Island, where some rounded masses of scoria; were embedded
in a conglomerate of no modem date. In a geological
point of view this circumstance allows us to consider
the grand linear train of ancient and modern volcanoes,
which fall on parallel fissures in the Andes, as extending
from lat. 55° 40' south to 60° north, a distence little less
than seven thousand geographical miles.
Perhaps the most curious feature in the geology of this
country, is the extent to which the land is intersected by
arms of the sea. These channels, as Captain King remarks,
are irregular and dotted with islands, where the granitic and
trappean rocks occur, but in the clay-slate formation are so
straight, that in one instance “ a parallel ruler placed on the
map upon the projecting points of the south shore, extended
across, also touched the headlands of the opposite coast.”
I have lieard Captain FitzRoy remark, that on entering
any of these channels from the outer coast, it is always necessary
to look out directly for anchorage; for further inland
the depth soon becomes extremely great. Captain Cook, in
entering Cliristmas Sound, had first .37 fathom, tlien 40, GO,
and, immediately afterwards, no soundings with 170. This
structure of the bottom, I presume, must arise from the
sediment deposited near the mouths of the channels, by the
opposed tides and swell; and likewise from the enormous
degradation of the coast rocks, caused by an ocean harassed
by endless gales.
The Strait of Magellan is extremely deep in most parts,
even close to the shore. About mid-channel eastward of
Cape Froward, Captain King found no bottom with 1536
fe e t: if, therefore, the water should he drained off, Tierra
del Fuego would present a far more lofty range of mountains
than it does at present. I will not here enter on any speculations
regarding the causes which have produced this
remarkable structure, in a district in which the latter movements
at least have been those of elevation. I may, however,
observe, that pebbles, and great boulders of various and
peculiar crystalline rocks, which have undoubtedly travelled
from the south-west coast, lie scattered over the whole of
the eastern part of Tierra del Fuego. One enormous block
of syenite near St. Sebastian Bay was barn-shaped, and had
a girth of 47 fe e t; it projected five feet above the sand,
and appeared to be deeply buried. The very nearest point
to which we can look for the parent rock, is about ninety
miles distant. On the shores of the Strait of Magellan, near
Port Famine, numerous semi-rounded fragments of various
granites and hornblendic rocks are strewed on the beach,
and on the sides of the mountain, to an elevation of thirty
or forty feet. Now to this point the high road from the
Southern and Western shores passes directly over the great
abyss of more than 1500 feet deep. Whatever may have
been the means of transport, it has not been one of indiscriminate
violence: for the two places, St. Sebastian Bay
and Shoal Harbour, where the great fragments are most
numerous, certainly existed previously to the last and
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