
- Parr ID. § vii] DETERMINATION OF ROCKS. 191 
_ viridite, olivine into serpentine, while secondary calcite, quartz, and 
zeolites run in minute veins or fill up insterstices of the rock (p. 107). 
6th, Is the rock a fragmental one; and if so, what is the nature of 
_ its component grains? (p. 105.) Is any trace of organic remains to 
be detected ? (p. 106.) 
ii. Chemical Analysis.’ 
The determination of the chemical composition of rocks by detailed 
analysis in the wet way, demands an acquaintance with practical 
chemistry, which comparatively few geologists possess, and is con- 
sequently for the most part left in the hands of chemists, who are not 
_ geologists. But as some theoretical questions in geology involve a 
_ considerable knowledge of chemical processes, so a_ satisfactory 
~ analysis of rocks is best performed by one who understands the nature 
of the geological problems, on whieh such an analysis may be ex- 
pected to throw light. As a rule, detailed chemical analysis lies out 
of the sphere of a geologist’s work; yet the wider his knowledge of 
chemical laws and methods the better. He should at least be able to 
employ with accuracy the simpler processes of chemical research, to 
some of which reference has already been frequently made. 
1. Pulverization—Much may be learnt regarding the composition 
of a rock by reducing it to powder. This may be roughly done by 
placing some pieces of the rock within folds of paper upon a surface 
of steel, and reducing them to powder by a few smart blows of a 
hammer. But a steel mortar is more serviceable. The powder can 
be sifted through sieves of varying degrees of fineness and the separate 
fragments may be examined with a lens. If they are dark in colour 
they may be placed on white paper, if light-coloured they are more 
readily observed upon a black paper. Portions of this powder may 
be carefully washed and mounted with Canada balsam on glass, as in 
the way already described for thin slices. Magnetic particles may 
be extracted with a magnet, the end of which is preserved from contact 
with the powder by being covered with fine tissue-paper. An electro- 
magnet will at once withdraw the particles of minerals which contain 
far too little iron to be ordinarily recognized as magnetic ; in this way 
the particles of a ferruginous magnesian mica may in a few seconds 
be gathered out of the powder of a granite. 
2. Treatment with Acid.—The geologist’s accoutrements for the 
field should include a small acid-bottle with a glass stopper prolonged 
downwards into a point. Dilute hydrochloric acid is commonly 
employed. When a drop of this acid gives effervescence upon a 
_ surface of rock, the reaction is caused by the liberation of bubbles of 
carbon dioxide, as this oxide is replaced by the more powerful acid. 
Hence effervescence is an indication of the presence of carbonates, and 
when brisk is specially characteristic of calcium carbonate. Lime- 
s Taken, with some alterations and additions, from the author’s Outlines of Field 
eology. 

