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It is composed of fine tufa, in which is imbedded a great number of large angular fragments of trachyte, some of which are more than three feet in diameter. It is the last bed but one, the surface being composed of lightly coherent strata of tufaceous ash, worn into an undulating surface by the action of the elements.
I brought away specimens of one of the tufaceous layers of a pinkish colour and it is a most extraordinary fact, that Professor Ehrenberg finds it almost wholly composed of matter which has been organized: he detects in it some siliceous-shielded fresh-water infusoria, and no less than twenty-five different kinds of the siliceous tissue of plants, chiefly of grasses.
I have much difficulty in believing that this thin layer of pitchstone flowed as lava. Eleventhly: sedimentary and tufaceous beds as before, passing into sandstone, including some conglomerate: the pebbles in the latter are of claystone porphyry, well rounded, and some as large as cricket-balls.
When these deposits occur in a flat tufaceous country like the present, a large space is devoid of vegetation, on account of the nitrates dissolving the tufa, and keeping it in a state unfavorable to the growth of plants. We found a great number of wells in this tufa. A place called Matlomagan-yana, or the "Links", is quite a chain of these never-failing springs.
At Jilmeca and some other points along the day's route the rock over which we passed was a white tufaceous material loaded with streaks of black flint. Sometimes this black flint passes into chert and chalcedony of blue and purple tints. Here and there, along the mountain sides, we caught glimpses of rock exposures, which looked snow-white in the distance.
On the contrary, in addition to old granitic and other igneous rocks, all explorers find only either innumerable undulations of sandstones, schistose, and quartzose rocks, or such tufaceous and ferruginous deposits as would naturally occur in countries long occupied by lakes and exuberant jungles, separated from each other by sandy hills, scarcely any other calcareous rocks being found except tufas formed by the deposition of landsprings.
In the upper and lower finer-grained strata, there were other concretions of a grey colour, containing calcareous matter, and so fine-grained and compact, as almost to resemble porcelain- rock: I have seen exactly similar concretions in a volcanic tufaceous bed in Chiloe. Gravel and boulders, 212 feet thick. b. Basaltic lava, 322 feet thick. c, d and e.
This is a specialty of all the district; throughout the Chocho towns, they make an excellent grade of palm-hats and everyone engages in the making. Both men and women braid palm, and in every yard there is excavated in the soft, tufaceous rock, a cueva, or cave, in which they work.
A lake at this great height, and without a barrier on any one side, is out of the question; consequently we must conclude that the tufaceous matter was anciently deposited beneath the sea. This volcanic formation, which I am informed by Mr.
The first is straight, with parallel sides, and about four feet wide; it consists of whitish, indurated tufaceous matter, precisely like some of the beds intersected by it.
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