MELLO: MICROSCOPIC A.L STRUCTURE OF ROCKS. 
165 
times the leaflets seem to consist of twisted or irregularly interwoven 
fibres. In sections of the chloritic schists, the chlorite often occurs 
in scales, which look like green glass cavities. Now and then hexa- 
gonal scales will be seen, which remain dark when the polariscope 
prisms are crossed. The polarisation colours of this mineral are 
feeble, blue and l)rown tints being the most prevalent, it is also 
decidedly dichroic. 
Hornblende is a very important constituent of some rocks. 
It occurs in distinct crystals, but is more often met with in crystalline 
masses and leaflets. Its structure is fibrous, and although it might 
at first sight be sometimes mistaken for biotite, the marked dichroism 
of the latter, as contrasted with the more feeble dichroism or rather 
pleochroism of the hornblende, will serve to distinguish between 
them. This will also enable us to distinguish between hornblende 
and augite. Transverse sections of the crystals of hornblende show 
two well defined sets of stria% which are its planes of cleavage, and 
intersect at an angle of 1 24^30 ', when tlie section is cut at right 
angles to the principal axis. Minute acicular crystals of this mineral 
are very frequently met with in the igneous rocks. Many adventi- 
tious enclosures occur in hornblende crystals, such as magnetite 
quartz, biotite, etc. 
Another very important constituent of some igneous rocks 
is Augite, It is constantly present in the dolerites and allied 
rocks. Under the microscope it is readily recognised, its colour is 
brownish or yellow, and it is ofton very distinctly crystallised. Its 
crystals are frequently very much cracked in an irregular manner, the 
result of strain during consolidation; their sections are usually eight 
sided, which may help us to distinguish between them and those of 
hornblende, which are six sided. The double refraction of augite is 
powerful, and in thin sections it polarises with brilliant colours, it is 
also dichroic, the dichroism being of a purplish tint. Augite crystals 
often CDntain numerous enclosures, such as biotite, leucite, magne- 
tite, etc. Occasionally the augite will be a mere cell-like crystal, 
filled up with a multitude of microscopic minerals. Augite also 
frequently occurs in the form of minute crystals, some curved, others 
club-shaped, or split into dichotomous points. 
