142 MINERALOGY AND LITHOLOGY. 



is beset partially disappear, for classification becomes its least interest- 

 ing feature. 



Methods of Study. 



In reference to the application of the microscope to this subject, suffi- 

 cient has already been said in the introduction to the first chapter, while 

 the optical properties of all of the minerals that are found as constituents 

 of our rocks have been indicated in describing them. In this chapter, 

 therefore, it will be assumed that no explanation of the optical effects 

 that are reproduced in the figures is necessary. Some of the figures 

 represent rocks as they appear with ordinary light, and hence need no 

 explanation. When crystals are represented as they appear in polarized 

 light, it is to be understood that the position of the Nicols is a determi- 

 nate one only in those cases where the planes of vibration of the light in 

 the Nicols are represented by cross lines on the figure; for, as a rule, 

 where nothing is to be gained by indicating the position of the axes of 

 elasticity in crystals, the Nicols have been so placed as to obtain colors 

 which would not complicate the hthography. I refer those again, who 

 wish systematic instruction on this subject, to the works of Zirkel and 

 Rosenbusch.* 



If, as has been shown, the microscope with ease and certainty deter- 

 mines minerals where chemical analysis fails, discovers ingredients in 

 rocks the presence of which was not before suspected, and brings to 

 the foreground many little circumstances which prove to be exceed- 

 ingly weighty, there are, too, cases where it fails. For example: in accu- 

 rately prepared sections from crystals of the triclinic feldspars, one can 

 easily determine the species ; but when the crystals lie scattered at hap- 

 hazard, though one can easily recognize them as being of triclinic feld- 

 spar, one cannot often determine the species, and in some such cases 

 chemical analysis is advantageously employed. Not long since the sci- 

 ence was largely dependent upon chemistry for its determinations, and 

 cumbersome methods for the separation and determination of ingredients 



* The work of A. von Lasaulx, "Elemente der Petrographie," is a condensed presentation of general lithol- 

 ogy, embracing the latest discoveries. The two volumes by Rosenbusch, on Milcroskopische Physiographic, 

 are very systematic and very valuable. The first volume treats of the minerals of importance in lithology, and 

 the second volume treats of the massive rocks. 



