156 MELLO: MICROSCOPICAL STRUCTURE OF ROCKS. 
Having thus carefully noted the character of the cavities found 
in the specimen under inspection, we may next proceed to determine 
the nature of its component minerals, most of which can be readily 
known by their structural appearance as seen with the microscope, 
either by the use of natural or by polarized light. 
Some minerals we shall at once recognize by their definite crys- 
talline forms, which will be apparent even without using the polari- 
scope. Thus sections of quartz crystals, the various specimens of 
felspar, augite and calcite may often be readily known by their charac- 
teristic shapes. Calcite, for instance, will be seen very clearly to be 
striated by fine lines, slightly iridescent, crossing each other and 
dividing the mineral into its primary rhomboidal crystals, the divi- 
sional planes of which are thus traced. But for minute investigation 
the polariscope is essential ; without it it would be utterly impossible 
to distinguish indefinitely shaped crystalline masses: also such 
minerals as hornblende, biotite and other micas and augite, as well as 
numerous less common crystalline substances would be frequently 
confounded. Without the polariscope we should be unable to discrim- 
inate between the different species of felspar ; we could not detect the 
difference between a crystalline and a glassy base, both would appear 
equally structureless. But directly the polariscope is used a new 
revelation is before us. Previously indefinite looking forms at once stand 
out in their true relationships, and the whole histor}" of the rock under 
examination may be read, each separate crystal, or part of a crystal, 
is seen sharply contrasted with its neighbour, and its species can be 
readily determined, the brilliant colouring due to the polarization of 
the light serving to bring to view details of structure which without 
its use would be either invisible or meaningless. 
In igneous rocks we have those that are glassy, such as obsidian, 
tachylite, etc., and crystalline rocks as granite and dolerite. How 
then to distinguish between a glassy rock and a crystalhne rock, 
when the characteristic features are not plainly visible to the eye ? A 
glass has no definite external form as has a crystal, nor is it of definite 
chemical character. In many rocks there is a glassy base enclosing 
crystals, whilst in others the base is crystalline, but the crystals are 
so small as to be invisible to the unaided sight. How is the glass to 
