1 8 MINERALOGY AND LITHOLOGY. 



2^. No sections behave as if isotropic, but all of them become dark, and no 

 longer distort the calcite figure when a crystallographic axis falls together with 

 the plane of vibration of the light. — Orthorhombic. Biaxial. 

 2^. For two of the crystallographic axes, this is no longer true. — Moiioclinic . Bi- 

 axial. 



2". For none of the axes is this true. — Tricliiiic. Biaxial. 

 II. Different parts of the substance act differently. In no position is the whole sec- 

 tion dark between crossed Nicols ; and the different parts bear no determinate relation- 

 ship to one another. — An aggregate. 



Circular Polarisation. Quartz is one of the commonest minerals that come under 

 microscopic examination. A basal section of quartz, as is well known, possesses the 

 property of circular polarization. Now, in microscopic sections, quartz is generally 

 cut so thin that the revolution of the light is too little to be recognized, hence quartz 

 behaves, in thin sections, like any other hexagonal substance. It is to be observed, 

 however, that sometimes it is not necessary to make very thin sections, and, in such 

 preparations, basal sections of quartz will not be entirely dark between crossed Nicols, 

 though they will not show the succession of the prismatic colors. 



Of course it will be understood that the preceding pages contain no 

 complete presentation of the principles involved in microscopic study. 

 Enough only has been said to make the figures accompanying this report 

 intelligible, and to draw the attention of those interested in the subject 

 to the principles in accordance with which a microscope must be modi- 

 fied, in order to do satisfactory study upon minerals and rocks ; modifica- 

 tion which, with little trouble, can be made upon any instrument, though 

 perhaps not with the accurate working of those instruments that are 

 made expressly for use in this now most important and fruitful study. 

 It will be borne in mind, too, that the determination of minerals is not 

 the only application of microscopic study, for most weighty conclusions 

 have been drawn from the arrangement of minerals with reference to 

 one another in rocks, and to the presence or absence of certain charac- 

 ters and ingredients, for the recognition of which any microscope will 

 suffice. 



The student of this department of mineralogy will find an extensive 

 literature, and, as it is mostly foreign, he will therefore find in this coun- 

 try a broad field for new investigation, where he will constantly be meet- 

 ing with new beauties and interesting facts. 



