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THE MICROSCOPE IN GEOLOGY. 
not to be distinguished by their optical properties, as in the case 
of those bodies which, as before mentioned, are translucent. 
When a mineral or rock under examination is entirely in the 
vitreous state, as, for example, obsidian, it appears when viewed 
under the microscope, merely as' a more or less transparent or 
coloured glass, presenting, if perfectly in the vitreous condition, 
no evidence of crystalline or other structure, except perhaps 
traces of the strise of viscid fusion. It is usually found on 
inspection, however, that some part of the mass is sufficiently 
de vitrified to allow of its structure and mineral composition 
being recognised. In some cases, when the glassy appearance 
presented to the eye would discourage any hopes of structure 
being discovered, the microscope proved the reverse most con- 
clusively ; as, for example, the section of glassy pitchstone from 
Arran, shown in PI. XVII. fig. 4, in which the pyroxenic and 
felspathic constituents of the rock are beautifully apparent, not- 
withstanding that the rock itself looks like so much dirty green 
bottle glass. 
In many cases, however, where the specimens are so perfectly 
in the vitreous state as to show no trace of structure whatsoever, 
this may be developed by carefully acting upon the surface by 
gaseous or liquid hydrofluoric acid. 
The rock sections may be prepared for the microscope as fol- 
lows : a fragment, from one-quarter to three-quarters of an inch 
square, and of convenient thickness, is chipped off the rock 
specimen in the direction of the required section, and ground 
down upon an iron or pewter plate in a lapidary’s lathe with 
emery, until a perfectly flat surface is obtained. This surface is 
then worked down still finer by hand on a slab of black marble, 
with less coarse emery, then upon a Water of Ayr stone with 
water alone, and, lastly, finished by hand with water on a 
slab of black marble. By these means the surface acquires a 
sufficient polish without being contaminated with rouge or other 
polishing powder or oil, as is sometimes the case with purchased 
sections of rocks. This side of the rock is now cemented by 
Canada balsam on to a small piece of plate glass about 1^- inch 
square and J- thick, which serves as a handle when grinding the 
other side on the emery plate as before ; this grinding is con- 
tinued until the section is so thin as to be in danger of breaking 
up from the roughness of the motion, upon which it is com- 
pleted, by further grinding with emery by hand on marble, 
and finished first upon Water of Ayr stone with water, and after- 
wards upon black marble, as before described. The section is 
now removed from the plate-glass, and mounted in Canada 
balsam on a slide, covering its upper surface with a thin glass 
as usual. 
The thickness to which such sections need be reduced is, of 
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