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The upper line of surface of the calcareous stratum, which is so conspicuous from being quite white and so nearly horizontal, ranges for miles along the coast, at the height of about sixty feet above the sea. The sheet of basalt, by which it is capped, is on an average eighty feet in thickness.
It is not, however, a true breccia, but a pudding-stone, composed, not of calcareous but of siliceous fragments; and these fragments are not angular, as in the true breccias, but rounded, indicating that they had been carried by water and consequently rounded by attrition. The connected pebbles must have been broken from rocks of great hardness to have withstood the effects of constant abrasion.
It is considerably harder than calcareous spar, but can be scratched with a knife; under the blowpipe it scales off, decrepitates, slightly blackens, emits a fetid odour, and becomes strongly alkaline: it does not effervesce in acids.
The animalcule that the marine polypus secretes live by millions at the bottom of their cells. Their calcareous deposits become rocks, reefs, and large and small islands. Here they form a ring, surrounding a little inland lake, that communicates with the sea by means of gaps. There they make barriers of reefs like those on the coasts of New Caledonia and the various Pomoton islands.
The road skirted with the bamboos above mentioned led us to the small village of San Fernando, situated in a narrow plain, surrounded by very steep calcareous rocks. This was the first Mission* we saw in America. Indian villages, governed by a priest, are called Pueblos de doctrina. The streets, which are wide and very straight, cross each other at right angles.
Serious damage may be done to the dyer by either of these classes of impurities, and I may tell you that the dissolved calcareous and magnesian impurities are the most frequent in occurrence and the most injurious. I told you that on boiling, the excess of carbonic acid holding chalk or carbonate of lime in solution as bicarbonate, is decomposed and carbonate of lime precipitated.
I had expected to see a calcareous fountain, but found the most magnificent masses of silica of infinite variety of form; shallow cones with cylindrical summits, pyramidal flights of steps, round basins with ribbed margins, and ponds of boiling water.
First, for the clay-slate formation: the slate is generally hard and bluish, with the laminae coated by minute micaceous scales; it alternates many times with a coarse-grained, greenish grauwacke, containing rounded fragments of quartz and bits of slate in a slightly calcareous basis.
These so-called slates are composed of a fine grained calcareous mud which has hardened into lithographic stone, and in which organic remains are almost as well preserved as they would be if they had been imbedded in so much plaster of Paris. They have yielded the Archæopteryx, the existence of which was first made known by the finding of a fossil feather, or rather of the impression of one.
The fragments of scoriae, embedded in the crystalline calcareous basis, are of a jet black colour, with a glossy fracture like pitchstone. Their surfaces, however, are coated with a layer of a reddish-orange, translucent substance, which can easily be scratched with a knife; hence they appear as if overlaid by a thin layer of rosin.
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