180 REV. J. MILNE CURRAN. 



rock, from the extreme acidic to the most basic, and from the 

 glassy to the most perfect, holo-crystalline condition. 



Methods of Research. 

 With few exceptions, I have collected my own specimens and 

 made my own slices and micro-photographs. I use Dick and 

 Swift's Petrological Microscope as the best form of instrument 

 with which I am acquainted for this special work. With this 

 microscope, the most minute crystals remain in the centre of the 

 field, while the Nicols rotate, either crossed, inclined, or parallel 

 under the highest powers. The change from plane to polarized 

 light, both parallel and convergent, is easily and rapidly effected. 

 A notice of this instrument will be found in the Proceedings of 

 the Royal Microscopical Society, June 1889, p. 432, or in the 

 Mineralogical Magazine, Vol. viii., p. 160. 



In dealing with basalt, a few simple micro-chemical and other 

 tests, besides purely optical ones, are necessary. Treating the 

 slice with hydrochloric acid is the only means, known to me of 

 separating augite from olivine in the fine micro-granular basis 

 (paste) of some basalts. 



The determination of the proportions in which particular 

 minerals are present in a rock can be effected in various ways. 

 More than forty years ago, Delesse* employed a simple, but reason- 

 ably accurate method, of determining the percentage of the mac- 

 roscopic minerals present in a rock. He polished a surface of the 

 rock to be investigated, and covered it with a thin paper. The 

 outlines of the minerals were then traced through with a pencil, 

 and the various minerals coloured differently. The tracing was 

 then gummed to a sheet of lead or tin-foil, and the outlines were 

 cut round with a scissors, and the pieces of the same tint were 

 sorted together. After removing the gum and paper, the frag- 

 ments were weighed, and, in this way, the proportion of each 

 mineral was ascertained. 



* Proeede mecanique pour determiner la composition des roches, Annales 

 des Mines, 4me. ser. tome xni. (1848), p. b7J. 



