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consisting of silica in a state more or less free from admixture with foreign 
substances. Then we have the crystals of mica and felspar, the most com- 
posite of mineral substances. These three substances are distinct from one 
another in crystalline and chemical composition. But then the micas and 
felspars adroit of the greatest and most puzzling varieties of chemical con- 
stitution ; one chemical element taking the place of another, without altering 
the crystalline character of the mica or the felspar in which the change of 
composition is found. We may have some conception of the composite 
structures and varieties of these minerals, when we state that nearly all, if 
not all, the metals and the mineral constituents of the sedimentary rocks may 
be found in the granites or other primary rocks. We have potash and also 
soda felspars: In the micas as well as the felspars we have not only the 
principal constituents, silica and alumina, but also soda, potash, lime, iron, 
magnesia, and water, replacing each other with most puzzling variations. 
We all know how gold is diffused through the quartz of some kinds of granite. 
The microscope is said also to reveal native iron among the constituents of 
granite. Doubtless all the metals and other minerals found in the cracks 
and crevices of the primary rocks were once in combination with these rocks. 
But I never could form any clear conception of the origin of metallic and 
mineral veins till I read Mr. Hopkins’s work on the subject. Very high 
geological and mineralogical authorities used to speak of gold as the most 
recent of all the metals ; — how more recent than others, I could not conceive. 
Some went so far as to imagine some recent geological event, when, as it were, 
a golden shower had fallen from heaven to earth ! The experiments of 
Daubree and Bischoff have proved the mechanical and chemical combination 
of water in granite. Though the authorities of the Geological Society were 
not convinced by Mr. Hopkins, their faith in the igneous origin of granite 
was first shaken, I believe, by my friend Mr. Clifton Sorby’s microscopical 
researches. By investigating microscopically the minute bubbles in crystals, 
he was able to determine whether the crystal was formed from an aqueous or 
some other liquid solution, or produced by cooling from a molten mass. 
With regard to Mr. Thompson’s assumption of the insolubility of silica in 
water, the geysers in Iceland afford a direct refutation of this. How, again, 
without the solubility of silica, can we account for the formation of silicified 
woods, without injury to the most delicate vegetable fibres ? Dr. Bowerbank 
has shown that the most delicate structures in sponges (which he had found 
destroyed by decomposition only a few hours after the death of the sponge), 
are faithfully and perfectly preserved in the flint. Before electro-metallurgy 
was discovered, we could form no idea as to the method nature takes to 
separate metals from the rocks through which they may be diffused. We 
have now, however, learnt the power of electricity in separating metals from 
the aqueous solutions of their salts. Soon after the discovery of this fact, a 
copper electrotype was produced without any artificial battery, by imbedding 
wires in two different strata of a mine, and using the galvanic current thus 
produced. Here then we have a demonstration of the electro-magnetic action 
of the earth, and of its power in the formation of mineral products. This 
