TO GEOLOGICAL RESEARCH. 71 



— like quartz^ leucite, felspar, etc. — the appearance presented 

 under the microscope is alike, their optical properties and the use 

 of polarised light afford the means of distinguishing between them 

 with certainty ; as also in the event of one substance being present 

 under two forms, as calcite from arragonite, monoclinic from 

 triclinic felspars, etc., and, in a similar manner, the structure, 

 whether crystalline or vitreous, is determined, and valuable 

 information gained, elucidating the mode of formation and origin 

 of the rocks themselves. 



For the purpose of investigating the optical properties of 

 minerals, various instruments have been devised. The apparatus 

 most commonly employed with microscopes consists of two 

 Nicol's prisms — one fitted beneath the stage of the microsope, and 

 the other, either above the eye-piece of the instrument or above 

 the objective, the lower one acting as the polariser, the upper one 

 as the analyser ; both polariser and analyser ought to be capable 

 of rotation. It is also necessary that there should be some means 

 of rotating the section itself between the Nicols, in order to ascer- 

 tain whether it is dichroic or not. 



A description of a few of the most important minerals which 

 enter into the composition of the eruptive rocks may not be 

 uninteresting. Felspars usually occur in long, prismatic crystals. 

 They are divided into two groups, according to the system under 

 which they crystallise — Orthoclase, in which the chief cleavages are 

 situated at right angles to each other ; Plagioclase, in which the 

 cleavage planes do not intersect at right angles. These two 

 groups may, in most cases, be distinguished under polarised light 

 by the differences they possess in their twinning, crystals of ortho- 

 clase and its varieties usually showing, when twinned, a median 

 divisional plane, on either side of which the halves of the crystal 

 depolarise the light in complementary colours ; whilst in the case 

 of plagioclase, the crystals exhibit numerous bands of different 

 colours. 



Sections of Quartz appear clear and pellucid under the micro- 

 scope ; they show circular polarisation, and often exhibit magnifi- 

 cent variegations of colour. Quartz often contains fluid cavities, 

 and sometimes contains bubbles of gas. 



Augite often occurs in dolerites and basalts in well-formed 



