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It is needless in this work to detail the arguments leading to this conclusion, derived from the form and the nature of the step-formed terraces on both sides of the valley, from the manner in which the bottom of the valley near the Andes expands into a great estuary-like plain with sand-hillocks on it, and from the occurrence of a few sea-shells lying in the bed of the river.
From the manner in which they cap the step-formed plains, worn by the sea within the period of existing shells, their deposition, at least on the plains up to a height of 400 feet, must have been a recent geological event.
I started with about fifteen attendants, on the 25th of May, for Lamteng, three marches up the Lachen. Descending the step-formed terraces, I crossed the Lachen by a good cane bridge.
The atolls of the larger archipelagoes are not formed on submerged craters, or on banks of sediment. Immense areas interspersed with atolls. Their subsidence. The effects of storms and earthquakes on atolls. Recent changes in their state. The origin of barrier-reefs and of atolls. Their relative forms. The step-formed ledges and walls round the shores of some lagoons.
At Port Desire and over a space of twenty-five miles inland, on the three step-formed plains and in the valleys, I everywhere passed over gravel which, where thickest, was between thirty and forty feet. Here, as in other parts of Patagonia, the gravel, or its sandy covering, was, as we have seen, often strewed with recent marine shells.
Southward of Port Desire, the plains have been greatly denuded, with only small pieces of tableland marking their former extension. But opposite Bird Island, two considerable step-formed plains were measured, and found respectively to be 350 and 590 feet in height. This latter plain extends along the coast close to Port St.
I spent some days in examining the step-formed terraces of shingle, first noticed by Captain B. Hall, and believed by Mr. Lyell to have been formed by the sea, during the gradual rising of the land. This certainly is the true explanation, for I found numerous shells of existing species on these terraces.
Near the north headland of the great Bay of St. At its south headland, 120 miles distant from the north headland, the 250 feet plain was again measured. Above this plain, towards the interior, Mr. Stokes informs me that there were several other step-formed plains, the highest of which was estimated at 1,200 feet, and was seen ranging at apparently the same height for 150 miles northward.
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