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Updated: May 24, 2025
The general nature of gravel is too well known to require description. The grains of quartz sand are either sharp cornered or else rounded pieces of stone of quartz, occasionally mixed with grains of other amorphous pieces of silica such as horn stone, silicious slate, carnelian, etc.; again, with lustrous pieces of mica, or red and white pieces of feldspar.
In the judgment of the writer, the phenomena presented by the Leadville ore deposits can be as well or better accounted for by supposing that the plane of contact between the limestone and porphyry has been the conduit through which heated mineral solutions coming from deep seated and remote sources have flowed, removing something from both the overlying and underlying strata, and by substitution depositing sulphides of lead, iron, silver, etc., with silica.
It is also known that some of these acids, which were called long ago by Thenard azohumic, are enabled to dissolve colloid silica in proportion to the nitrogen which they contain. In the formation of these latter acids worms probably afford some aid, for Dr. H. Johnson informs me that by Nessler's test he found 0.018 per cent. of ammonia in their castings.
Silica is the mineral with which the waters of this fountain are impregnated, and the substance which they deposit, as they slowly evaporate, is named siliceous sinter. Of this material is composed the mound, six or seven feet high, on which the spring is situated.
Often in that nameless discouragement before unfinished tasks, unfulfilled aims, and broken efforts, I have thought of how the creative Word has fashioned the opal, made it of the same stuff as desert sands, mere silica not a crystallized stone like a diamond, but rather a stone with a broken heart, traversed by hundreds of small fissures which let in the air, the breath, as the Spirit is called in the Greek of our New Testament; and through these two transparent mediums of such different density it is enabled to refract the light and reflect every lovely hue of heaven, while at its heart burns a mysterious spot of fire.
Silica and alumina combine to make common clay, but alumina forms itself into the red ruby, the golden-tinted topaz, the violet oriental amethyst, the red, white, yellow, and violet sapphire, and the beautiful green emerald. With substances of such rare capabilities we may expect rich results in color and form. We turn now to deposits from water of these two substances, especially the first.
From the close similarity in so many respects, between the obsidian formations of Hungary, Mexico, Peru, and of some of the Italian islands, with that of Ascension, I can hardly doubt that in all these cases, the obsidian and the sphaerulites owe their origin to a concretionary aggregation of the silica, and of some of the other constituent elements, taking place whilst the liquified mass cooled at a certain required rate.
Causticity and fluidity have long since been excluded from the characteristics of the class, by the inclusion of silica and many other substances in it; and the formation of neutral bodies by combination with alkalis, together with such electro-chemical peculiarities as this is supposed to imply, are now the only differentiæ which form the fixed connotation of the word Acid, as a term of chemical science.
Were we to admit that the bitumen was sufficiently fluid to penetrate all parts of the vegetable debris, as silica and carbonates of lime and iron have done in so many cases, we should meet with one great difficulty.
Thus, we may imagine the sides of a fissure to be incrusted with siliceous matter, as Von Buch observed, in Lancerote, the walls of a volcanic crater formed in 1731 to be traversed by an open rent in which hot vapours had deposited hydrate of silica, the incrustation nearly extending to the middle.
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