72 THE MICROSCOPE IN GEOLOGICAL RESEARCH. 



crystals. The colour, in ordinary transmitted light, varies from a 

 dark purple-brown to a pale brownish-yellow ; they are transparent, 

 and exhibit fine colours under polarised light. Hornblende is the 

 only mineral likely to be mistaken for Augite. The form of the 

 crystals is slightly different, and it is distinctly dichroic, which 

 Augite is not. 



Olivine, in very thin sections, under the microscope, appears 

 almost colourless, and of a light greenish tint in those of 

 moderate thickness ; it polarises in tolerably strong colours, red 

 and green, but not so brilliant as those displayed by quartz. The 

 surfaces of Olivine are nearly always rough, since the ordinary 

 grinding is never capable of imparting a smooth polished face to 

 the section. It is frequently altered into serpentine. 



Mica varies in colour from silvery w^hite to almost black, 

 according to the kind. One of the most common forms in dole- 

 rites is Biotite, which is black or dark-green, frequently occurring 

 in irregular polygonal plates or narrow strips, containing well- 

 marked parallel lines. The colour varies under polarisation from 

 pale brown to dark opaque-brown, or nearly black. 



Magnetite occurs in black grains, which are quite opaque in 

 the thinnest sections. The most frequent crystalline form is the 

 octahedron, its section being a black square. 



Granites, dolerites, and basalts, are chiefly made up of some 

 of these minerals in varying proportions. 



In the foregoing brief historical sketch of the application of 

 the microscope to geological investigation, and the indication of 

 the several lines on which that investigation has been carried on, 

 it has been my object to show how interesting the study of micro- 

 scopic geology may be, even to amateurs^ who have neither the 

 time nor the knowledge to enter into original researches concern- 

 ing the conditions under which our rocks have been formed, but 

 who will thus be able better to appreciate the work, and accept 

 intelligently the conclusions of those scientific men who have 

 devoted themselves to the elucidation of these subjects. 



