aye 
1908 “-) S@EOGNOSY.” 70" RGpaaeEe 
cases it has been found possible to establish characteristic distinctions 
for individual mineral species, by noting the angle between the 
direction of their extinction and certain principal faces. It would be 
beyond the scope of this volume to enter into the details of this 
subject, which must be sought in some of the works already cited. 
The publications of Zirkel, Rosenbusch, von Lasaulx, Fouqué and 
Michel-Lévy may especially be consulted. 
Pleochroism (Dichroism).—Some minerals show a change of 
colour when a Nicol prism is rotated below them, hornblende, 
for example, exhibiting a gradation from deep brown to dark 
yellow. A mineral presenting this change is said to be pleochroic 
(polychroic, dichroic, trichroic). To ascertain the pleochroism of 
any mineral we may remove the upper polarizing prism and leave 
only the lower. If, as we rotate the latter directly under the stage 
of the microscope, no change of tint can be observed, there is no 
pleochroic mineral present, or at least none which shows pleochroism 
at the angle at which it has been bisected in the slice. But we 
may often detect in a slice of some crystalline rock little crystals 
which offer a change of hue as the prism goes round. ‘These are 
examples of pleochroism. This behaviour may be used to detect 
the mineral constituents of rocks. Thus the two minerals horn- — 
blende and augite; which in so many respects resemble each other, 
cannot always be distinguished by cleavage angles, in microscopic 
slices. But as Tschermak pointed out, augite remains passive or 
nearly so as the lower prism is rotated: it is not pleochroic, or 
only very feebly so; while hornblende, on the other hand, es- 
pecially in its dark varieties, is usually strongly pleochroic. It is to 
be observed, however, that the same mineral is not always equally 
pleochroic, and that the absence of this property is therefore not so 
reliable as a negative test, as its presence is as_a positive test. 
In his examination of rocks with the microscope the student may 
find an advantage in propounding to himself the following questions, 
and referring to the previous pages here cited. 
Ist, Is the rock entirely crystalline (p. 105) consisting solely of 
crystals of different minerals interlaced ; and if so, what are these 
minerals? 2nd, Is there any trace of a glassy ground-mass or base 
(p. 99)? Should this be detected, the rock is certainly of volcanic 
origin (p. 104). 38rd, Can any evidence be found of the de- 
vitrification of what may have been at one time the glassy basis of 
the whole rock? ‘This devitrification might be shown by the appear- - 
ance of numerous microscopic hairs, rods, bundles of feather-like 
irregular or granular aggregations (p. 100). 4th, In what order 
did the minerals crystallize? This may often be very clearly 
made out with the microscope, as, for instance, where one mineral 
is enclosed within another (p. 99). 5th, What is the nature of 
any alteration which the rock may have undergone? In a yast 
number of cases the slices show abundant evidence of such meta- 
morphism ; felspar passing into granular kaolin, augite changing into 

