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First Part.

SECTION FOURTH.

OF PORPHYRIES AND GRANITES.

Before occupying ourselves with these costly stones, we will say a few words respecting the primitive material which enters into their composition.

OF QUARTZ.

§ 45. Quartz is the first of the primitive glasses.-This same material is supposed to form the great internal rock of the globe; its exterior portions, which form the base and nucleus of the highest elevations of the earth, are composed of the same primitive matter. The nucleus of these mountains became at first surrounded with, and covered by broken fragments of this glass, together with scales of jasper, spangles of mica, and little crystallized masses of feld-spar and shorl, from which were formed, by their union, huge masses of granite, porphyry, and all other vitreous rocks.

To perfectly understand the nature and formation of porphyry and granite, we must first define the difference between quartz and jasper.

OF JASPER.

§ 46. Jasper is simply a quartz, more or less filled with metallic particles; these color it, and render its fracture less clear than that of quartz; it is also more opaque. Yet as jasper, with the exception of its color, is only composed of a single substance, it may be regarded as a species of quartz, unmixed with anything but metallic vapors.

§ 47. Mica is a material, the substance of which is nearly as simple as that of quartz or jasper, and all three are of the same nature. Its formation is contemporary with that of these two glasses. It is not found, like them, in large, hard, and solid masses, but generally spangles or small thin plates, and disseminated through other vitreous substances. These spangles of mica have finally formed talcs, which are of the same nature, but with larger laminę talcs or plates. Usually, small parcels of matter come from those which are in larger masses. Here, on the contrary, the large volume of talc is formed from particles of mica which first existed, and the molecules of which, being united by means of water, have formed talc in the same manner as sandstone is produced by the blending of quartzose sand.

§ 48. Common Talc is a kind of unctuous stone, soft, clean, pearl-colored, and easily separated into plates, which, when thin, are quite transparent. It is easily bent or cut, is greasy and fatty to the touch, is broken with difficulty, will bear a strong heat without suffering much change, and is not dissolved by any acid or alkaline menstruum in a liquid form. The most esteemed talc is that which is as transparent as clear water, that with a green tint is not as valuable.

Talc is prepared for commerce by splitting it into plates with a thin, two-edged knife, so that the back of the plate may not chip off. This is used throughout Siberia for windows and lanterns instead of glass, and no glass is clearer or more transparent than good talc.

Some talcs are greenish, yellow, and even black, and these colors, which affect their transparency, do not change their other qualities; these colored talcs are nearly as soft to the touch, and pliant to the hand, and resist, like the white talc, the action of acids and fire.

It is easy to understand, from these facts respecting the composition of porphyry and granite, why they have a greater hardness and solidity than Marble; yet these are not the only materials which have aided in their foundation.

§ 49. Feld-spar is also a vitreous matter, and is sparry when broken; it is never found in large masses like quartz or jasper, but in small crystals incorporated in granite and porphyry; sometimes, also, in little isolated pieces in the purest clay, or in the sands formed from the decomposition of porphyry or granite, it being one of the constituent parts of these rocks. It is usually found crystallized and colored, and in small masses.

Feld-spar is sometimes opaque, like quartz, but oftener almost transparent. The different tints of red or violet with which its crystals are often colored, indicate a strong proximity between the time of its formation and that of the metallic sublimations which have penetrated and tinged jaspers of different colors.

§ 50. Shorl is the last of the five primitive glasses, and, as it has several characteristics in common with feld-spar, it can be plainly seen, by contrasting them, that both have a common origin, and that both were formed at the same time, and by the same law of nature, during the general vitrification.

Shorl is a sparry glass-that is, composed of longitudinal plates like the feld-spar; it is also found in small crystalized masses, its crystals forming prisms surmounting pyramids.

This explained, we are naturally led to speak of porphyry. Quartz, jasper, mica, feld-spar, and shorl are, as we have just seen, the simplest substances which nature has produced by means of fire. We will now follow the combinations she has made by the mixture of two, three, four, and sometimes by the whole five together, to compose other substances by the same means of fire, at the time of the first consolidation of our globe. This seems to be a digression from our subject, but it is that we may penetrate it more deeply, and to initiate the Marble workers into this branch of the art, which is usually but little understood by them.

§ 51. Porphyry is the most precious of these composite substances, and, after the jasper, the most beautiful of all vitreous matter found in large masses. It is, as we have just said, composed of jasper, feld-spar, and small particles of Shorl, mingled together. It cannot be confounded with the jaspers, they being of a simple substance, containing neither feld-spar or shorl; nor can it be classed with the granites, as they never contain jasper, but are made up of three or four other substances, namely, quartz, feld-spar, shorl, and mica.

The name of prophyry seems exclusively to designate a purplish red substance, which is, in fact, the color of the finest porphyry; but this title is extended to all porphyries, without distinction of color.

The red porphyry is interspersed with very small spots, more or less white, sometimes reddish. These spots are the particles of feld-spar and shorl which were disseminated and incorporated into the past of the jasper. The essential characteristics of all porphyries, and by means of which they can always be recognized, is this mixture of feld-spar or shorl, or of both together, with the substance of the jasper; they are more opaque and highly colored when jasper enters largely in their composition, and become somewhat transparent by the presence of a larger quantity of feld-spar. The less opaque the porphyry, the harder it is, while, on the contrary, the more transparent the Marble, the softer is it found to be.

In porphyry the ground or paste is deeply colored; the grains of feld-spar and shorl are white, or sometimes of the color of the ground, but of a much paler shade; in granite, on the contrary, the feld-spar and shorl are colored,-and the quartz, which may be regarded as the paste, is always white. This proves that the substance of jasper is the base of porphyry, as quartz is that of granite.

§ 52. Although much less common than granite, porphyry is often found in masses, and in some places in large blocks. It is usually a neighbor of the jasper, and both rest, like the granite, upon quartzose rocks; this proximity indicates a contemporary formation.

The durable solidity of the substance porphyry also proves its affinity with the jasper-neither tarnish, except by a long-continued action of watery elements, and of all substances in the world used in large quantities, quartz, jasper, and porphyry are the most unchangeable.

Black porphyry, properly called, has an entirely black ground, with small, oblong spots, and only differs from the red porphyry in color.

There is a porphyry with a brown ground, with large, greenish oblong spots; another is also found with a reddish brown ground, with spots of bright green, and others of a blackish brown ground, with spots of blackish and greenish tints.

The green porphyry has several varieties; the green-antique Serpentine, the ground of which is green, and the spots oblong or parallelopipedon, is of a bright of pale green, and partaking of the nature of feld-spar or shorl.

We will now pass to the second part of this section, which, though offering less interest to our curiosity, possess for us a much greater degree of utility.

OF GRANITES.

§ 53. Of all matter produced by the primitive fire, granite is the least simple and the most varied; it is usually composed of quartz, feld-spar, and schorl; quartz, feld-spar, and mica, or of quartz, feld-spar, shorl, and mica.

The red tints of feld-spar and the blackish brown of shorl are, doubtless, attributable to the metallic sublimations, which, in the same manner, have colored the jasper, and which permeated the matter of feld-spar and shorl when in a state of fusion. However, all are not thus colored, as white and whitish feld-spars and porphyry, feld-spar is not distinguishable from quartz by its color.

The Vosges, though not the highest elevations, are composed of granite, exhibiting no vestige of marine products, and these granites are not covered with calcareous beds, although the sea has borne its relics to much greater heights in other places. With this exception, it is only in high vitreous mountains that the ancient structure and primitive composition of the earth can be seen bare in masses of quartz, veins of jasper, groups of granite, and metallic veins.

When the metallic exhalations are abundant, and also mixed with the other corrosive elements, they deteriorate the substance of the granite in time, and even change that of quartz; this is seen in the sides of all perpendicular clefts in which veins of metallic mines have been found-the quartz seems decomposed, and the adjacent granite is crumbly.

Buffon says, with reason, that granite is only found on high mountains, or at the foot of them, having been precipitated by time, or detached by waters. An important historical fact supports this theory; the discovery in a marsh of the piece of granite which serves for the base of the colossal statue of Peter the great. This fragment belonged to no mass, and the neighboring mountain was surmounted by pieces of the same nature.

§ 54. M. Huot also says, that granite is styled by mineralogists, a rock composed of lamellar feld-spar, quartz and mica, almost equally disseminated. If the mixture is equal, common granite is found.-When the granite contains crystals of feld-spar of a regular form, and larger than those of the other constituents, it takes the name of porphyroide granite, because, at first sight, it bears the aspect of porphyry.

It would be very interesting to follow the thought suggested by Buffon for the study of granite, and to avail ourselves of the reasoning of M. Huto; but this would take us too much from our subject, and we should occupy ourselves here less with the means used by nature in the composition of these Marbles and granites, than with those which may enable man to avail himself of a part of them.

Nowhere, says Bexon, quoted by Buffon, can one conceive a more magnificent idea of masses of granite, than in the mountains of the Vosges. In a thousand places they offer blocks much larger than those we admire in the most superb monuments, and the broad summits and steep sides of these mountains are nothing but piles and groups of huge granite rocks, heaped upon each other.

§ 55. Since the epoch in which these naturalists wrote, many surveys have been made, and we can say with them, that the Vosges contain the greatest wealth of this kind; that they produce very fine granites of various grains and colors, several species of porphyry and richly colored jaspers.

The use of granite in the arts would be highly valuable for many purposes, particularly for water ducts, troughs, basements of manufactories, pedestals of funeral monuments, pavements or curbstones, props, supporting columns, and even for articles requiring polish; for the most of the Vosgean granites take polish as well as marble, and retain it for a long time.

The principal objection is the difficulty of working them. There is the certainty, at least, that neither frost, or water, or the sun or moon, which acts so powerfully on stone, can destroy the article which the hand of the artist has formed. There is also the certainty that troughs, fountains, and vessels for holding water, will not leak, and that the necessity will be obviated, of cementing pieces, which is often done in blocks of our best stone.

§ 56. Marble workers use granite but little, on account of its hardness. This is not their fault, but that of persons who do not take this difficulty of labor into consideration. A little reflection will convince one that the transportation and setting being of the same price, it is more advantageous to build works that will endure forever, without the trouble of repairing and renewing, than those that last but a brief time.

We will not speak farther upon the uses of granite, as much that we would say on this head would be a repetition of what we have already said respecting Marble.



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