766 MICROSCOPIC PETROLOGY. 



rock-mass, tending to elucidate its formation and origin." The 

 mode recommended by Mr. D. Forbes of making Transparent 

 Sections of Rocks for Microscopic examination, is essentially the 

 Bame with that already described (§§ 138-140). 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 einery, until a perfectly flat surface is 

 obtained. This surface is then worked down still finer upon a slab 

 of Black Marble, with less coarse emery, then upon a Water of Ayr 

 stone with water alone, and lastly polished with water on a slab of 

 Black Marble. The polished surface being then cemented to a slip 

 of Plate-glass, the other surface is to be worked down in the same 

 manner, until the section is reduced to a sufficient thinness ; when it 

 is to be transferred to a slide, and mounted in Canada balsam in 

 the usual mode. The examination of such a Rock-section enables 

 a Mineralogical analysis to be made even of the most compact and 

 apparently homogeneous rock ; for even when the glassy appearance 

 of a vitrified rock would discourage any hopes of structure being 

 discovered, some portion may generally be found by Microscopic 

 examination, in which the vitrification is so far from being complete 

 as to enable the component Minerals to be distinctly recognized. 

 Thus in a specimen of Glassy Pitchstone examined by Mr. Forbes, 

 the Pyroxenic and Feldspathic constituents of the rock were 

 beautifully apparent, notwithstanding that the rock itself looks 

 like so much dirty green bottle-glass. And in many cases in which 

 the specimens have been so perfectly vitrified as to show no trace 

 of structure in the first instance, this may be developed by carefully 

 acting upon the surface by gaseous or liquid Hydrochloric acid. 

 Frequently, again, Mineral constituents are thus discovered, whose 

 existence had been previously unsuspected from their being too 

 minute to be recognized by the eye ; and the presence of these may 

 have a most important bearing upon the question of the mode in 

 which the Rock-masses have originated. Thus it has been shown 

 by Mr. Sorby that the Quartz of Granites contains Water in 

 numerous minute cavities excavated in its solid crystals ; which 

 shows that Granites have solidi6ed at a heat far below the fusing 

 points of their constituent minerals, and at such a pressure as to 

 enable them to entangle and retain a small amount of aqueous 

 vapour. Similar cavities have been detected by Mr. Sorby not 

 merely in the Quartz of Volcanic rocks, but also in the Felspar 

 and Nepheline ejected from the crater of Vesuvius; and this fact 

 renders it probable that the two classes of rocks were formed by 

 identical agencies, as might be concluded from the general arrange- 

 ment of their Mineral components. For it is affirmed by Mr. D. 

 Forbes that ''the Microscopic examination already made of many 

 hundred sections of Eruptive rocks, differing widely in Geological 

 age and Geographical distribution, shows that in all rocks of this 



