EXAMINATION OF MINERALS IOSl 



experienced eye can generally tell the difference, for the latter 

 appears more diaphanous than the former. In petrology, as in 

 medicine, a cautious empiricism, which signifies experience concen- 

 trated and regulated by common sense, is sometimes even more 

 valuable than any amount of printed rules. 



On this account the student may be glad to have a few general 

 directions as to the best method of studying a rock slice. First, 

 look at it with a rather strong pocket lens, especially if it be crystal- 

 line or fragmented, so as to get a good idea of its general structure, 

 which is sometimes less easily seei> under the microscope, because the 

 field of view at any one time is small, and high magnification may 

 make it ' hard to see the wood for the trees.' Then place it on the 

 stage and examine first with transmitted, next with reflected light. 

 The former shows what minerals are colourless, and the natural 

 tints of the coloured, bringing out well slight differences of structure, 

 especially any due to incipient decomposition. 1 The latter enables 

 him to distinguish the opaque minerals, e.g. pyrite from magnetite, 

 sometimes the latter from other iron oxides ; to identify native iron, 

 awaruite, and gold ; perhaps also graphite, but it is better to verify 

 the last by powdering a little of the rock, when the streak is easily ob- 

 tained. Sometimes we are helped in distinguishing even transparent 

 minerals by the different way in which they reflect light. Next, 

 put on the polariser and examine pleochroism ; and lastly, insert the 

 analyser, for the general study of the tints produced and especially 

 of the extinction angles of certain of the minerals. When a mineral 

 gives very low polarisation tints, especially in the case of certain 

 aggregates, or we are searching for a glassy base in a slice crowded 

 with microliths, we may be helped by inserting a selenite or quartz 

 plate (better just below the slide) to obtain a coloured field, 2 for the 

 eye can be more sure of a difference of tint than of a very faint 

 glimmer of light. 



In dealing with rocks apparently clastic we have to determine 

 whether the structure is original, or has been superinduced (by crushing 

 or shearing) ; also what amount of mineral change has subsequently 

 occurred, and of what this is significant investigations which, 

 though of the highest interest, are often by no means easy, so that 

 the most experienced worker may occasionally be baffled. One final 

 piece of advice : before adopting a conclusion, look at it all round, 

 to see how it fits in with previously acquired knowledge and the 

 probabilities in the particular case. 



The micro-spectroscope has not at present been so much used 

 by petrologists as it might have been. It has been employed 

 by Professor Orville Derby in the determination of the pre- 

 sence of monazite in Brazilian sands. 3 This mineral contains a 

 large percentage of didymium, and accordingly gives the bands 



1 Holes in the slice and bubbles in the balsam, which often perplex beginners, 

 are now most readily detected. Also a mineral of easy cleavage is sometimes slightly 

 ruptured in the grinding, producing diffraction tints (as in calcite). These, between 

 crossed nicols, might be mistaken for oscillatory twinning ; but at the present stage 

 their true nature is obvious. 



2 This method can also be used to enhance a weak pleochroism. 



3 American Journal of Science, vol. xxxvii. 1889, p. 109. 



