246 OUTLINES OF FIELD-GBOLO mi 



he is most likely to meet with c'ither as principal or as 

 accessory ingredients of rocks. By carefully crushini; 

 in a steel mortar a small fragment of such a mineral lie 

 will obtain an abundant supply of minute cleavage- 

 flakes which he ran wash out and mount as slides for 

 constant study and reference, until he has Tamil i. 

 himself with the microscopic characters of the mini-nils, 

 and prepared himself for the recognition of tin 

 minerals as components of rocks. He may also increase 

 his knowledge by obtaining carefully prepared thin 

 of charactersitic minerals of each system, cut with 

 ence to the crystallographic axes of the crystals. 



By training his eye in the study of such slides he will 

 fit himself for discriminating the minerals as they occur 

 in rocks. But he ought on no account to speak con- 

 fidently about the microscopic structure of rocks until he 

 feels assured that the confidence arises from sound know- 

 ledge, and should specially avoid rushing into print on 

 the subject. 



THE MICROSCOPE 



As already stated (ante, p. 32), it is not necessary to 

 procure an expensive microscope with very high magni- 

 fying powers. For most purposes of the field-geologist 

 the xi-inch objective with a magnifying power of from 

 20 to 50 or 60 diameters, according to the eye-pieces 

 employed, will be found the most generally useful. But 

 he should also have an objective capable of giving, with 

 suitable eye-piece combinations, magnification up to from 

 200 to 300 diameters. A nose-piece for both objectives 

 screwed to the foot of the tube saves much time and 



