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Updated: May 24, 2025
Macie to believe that they consisted principally of silica, but that before calcination some vegetable matter must have been present. A determination of the specific gravity of the substance by Mr. Macie gave 2.188 as the result. Another determination by Mr. Cavendish gave 2.169.
We think of something in the organic that lifts and moves and redistributes dead matter, and builds it up into the ten thousand new forms which it would never assume without this something; it lifts lime and iron and silica and potash and carbon, against gravity, up into trees and animal forms, not by a new force, but by an old force in the hands of a new agent.
The strongest and most lasting cement is siliceous, and sand rocks whose grains are closely cemented by silica, the chemical substance of which quartz is made, are known as quartzites. We are now prepared to understand how sandstone is affected by the action of the weather.
CONCRETIONS. Ground water seeping through the pores of rocks may gather minerals disseminated throughout them into nodular masses called concretions. Thus silica disseminated through limestone is gathered into nodules of flint. While geodes grow from the outside inwards, concretions grow outwards from the center. Nor are they formed in already existing cavities as are geodes.
Silica is one of the cheapest and most abundant materials of all the earth's products, so its combination with iron will greatly lengthen the life of the iron supply; and it is probable that in the future combinations of other materials will yield better and cheaper metals than any thus far produced. The amount of metal which can be reworked is constantly increasing.
It was readily presumed by geologists that these masses were formed by a chemical aggregation of particles of silica, originally held in solution in the mass of the chalk. But whence the silica in a substance so different from it?
As she spoke, the two men stepped out of the "testing shed" the huge structure that housed their Osnomian-built space-cruiser, "Skylark II." Seaton waddled clumsily, wearing as he did a Crane vacuum-suit which, built of fur, canvas, metal and transparent silica, braced by steel netting and equipped with air-tanks and heaters, rendered its wearer independent of outside conditions of temperature and pressure.
Before seeking to describe the diverse colors made largely by one substance, let us remember that while silica, the principal part of these water-built mounds, is one of the three parts of granite, namely, the white crystal quartz, it is also the substance of the beautifully variegated jasper, the lapis lazuli, the green malachite, and the opal, with its cloudy milk-whiteness through which flashes its heart of fire.
The difference also of the lavas poured out from the same mountain at two distinct periods, especially in the quantity of silica which they contain, is often so great as to give rise to rocks which are regarded as forming distinct families, although there may be every intermediate gradation between the two extremes, and although some rocks, forming a transition from the one class to the other, may often be so abundant as to demand special names.
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