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Updated: April 30, 2025
Hence we may conclude what must be the heat in this solfatara at the depth of thirty or forty fathoms. The sulphur reduced into vapour is condensed into fine crystals, which however are not equal in size to those M. Dolomieu brought from Sicily. They are semi-diaphanous octahedrons, very brilliant on the surface, and of a conchoidal fracture.
Everybody knows a piece of cannel coal when it is seen, how it appears almost to have been once in a molten condition, and how it breaks with a conchoidal fracture, as opposed to the cleavage of bituminous coal into thin layers; and, most apparent and most noticeable of all, how it does not soil the hands after the manner of ordinary coal.
They pass into pitch-stone; and we may consider them as porphyries with a basis of obsidian. The second variety is found in fragments much less considerable. It is in general of a greenish black, sometimes of murky grey, very seldom of a perfect black, like the obsidian of Hecla and Mexico. Its fracture is perfectly conchoidal, and it is extremely transparent on the edges.
In nine or ten miles to the north of Boorbarna, we crossed a curious tract of country, covered with what I considered a variety of quartz, which breaks with a conchoidal fracture, but it has very much the appearance of flint; in many places the pieces were large, with sharp angles; my sons complained that it injured their horses' feet, but by alighting, and leading our horses over the worse parts, I did not perceive any bad effects from it.
A. The diameter is 7 feet and the pitch 14 feet. The propeller is Holm's conchoidal propeller. Its diameter is smaller than is advisable, being limited by the draught of water of the vessel; and the vessel was required to have a small draught of water to go over a bar. This engine makes, under favorable circumstances, 100 strokes per minute.
There were built-up fabrics, called Charlottes, caky externally, pulpy within; there were also marangs, and likewise custards, some of the indolent-fluid sort, others firm, in which every stroke of the teaspoon left a smooth, conchoidal surface like the fracture of chalcedony, with here and there a little eye like what one sees in cheeses.
These things were not cut and polished, but made by chipping or cracking off pieces from a lump. This one can see by the traces of conchoidal fracture which they all show. The art is not wholly understood, for it perished soon after the Conquest, when iron came in; but, as far as the theory is concerned, I think I can give a tolerably satisfactory account of the process of manufacture.
Climbing the Rastrojito slopes, we sighted the Pedras Negras: these are huge travelled rocks of basalt, jet-black, breaking with a conchoidal fracture, and showing debris like onion-coats about their base. The aspect was fantastic, resembling nothing so much as skulls 10 to 15 feet high.
Sometimes it is very pure, very homogeneous, of a dusky olive-green, and of a conchoidal fracture: sometimes it is veined, mixed with bluish steatite, of an unequal fracture, and containing spangles of mica. In both these states I could not discover in it either garnets, hornblende, or diallage.
A fortunate chance led us to observe some very curious geological phenomena, which we again met with in the Cordilleras of Mexico. The limestone of Barcelona has a dull, even, or conchoidal fracture, with very flat cavities. It is divided into very thin strata, and exhibits less analogy with the limestone of Cumanacoa, than with that of Caripe, forming the cavern of the Guacharo.
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