Anniversary Address of the President, H. C. Sorby, FBS. 129 
Grains yoloo Of an inch in diameter show only the pale bluish 
white of the first order, or the complementary brown. 
Calcite, &e.—Crystallized calcite may be at once distinguished 
from quartz by the more or less conspicuous planes of cleavage. 
This will not, however, apply in the case of fragments of calcareous 
organic bodies. Since the index of refraction is considerably greater 
than that of Canada balsam, their outlines are tolerably well marked, 
when mounted in that substance. ‘he action of calcite and arragonite 
on polarized light being so powerful, the bright orders of colours 
are best seen when the fragments are about go'o> of an inch in 
diameter, and when more than yoyo we usually obtain the faint 
reds and greens of high orders or merely white light, distinguished 
from the bluish white of the first order by not giving the comple- 
mentary brown. When examined in water all doubt can be re- 
moved by observing the action of dilute hydrochloric acid, which 
dissolves calcite and arragonite with effervescence, and leaves quartz 
and many other minerals unchanged. 
Mica.—tThis is best recognized by its occurring as thin plates 
haying a laminar structure. When in water or mounted in balsam 
the flat surfaces lie parallel to the supporting glass, and it may be 
somewhat difficult to appreciate their thickness and to distinguish 
them from flat plates of quartz. If they cannot be made to turn 
round, so that their edges may ‘be seen, the only course that can be 
adopted is to illuminate carefully and observe the efiects of slight 
changes in focal adjustment, which may suffice to prove that the 
fragments are flat and have a laminar structure, with no cleavage 
in any other direction. The occurrence of small granules or crystals 
of red oxide of iron between the lamin may sometimes greatly 
assist in forming a satisfactory conclusion, since a perfectly flat 
thin layer of any such material is not at all likely to occur in 
quartz. Advantage may also be taken of the difference in refractive 
power, as described below when treating on glassy felspar. 
Hornblende, &e.—What I have said when describing the horn- 
blende and schorl in decomposed rocks will, I think, sufficiently 
explain the methods I have employed in identifying them in 
deposits. Perhaps the most decided difference between coloured 
hornblende and schorl is the intense dichroism of the latter, so 
that in certain positions no light passes through it. 
Felspars.—As a general rule the difference between the felspars 
in granitic and in volcanic rocks is so great that it may be con- 
venient to consider them separately. Those in granitic rocks are 
almost or quite free from cavities, or at most contains only a few 
fluid cavities, whereas the glassy sanidin of volcanic rocks often con- 
tains many well-marked glass cavities. It also differs greatly from 
other felspars in various ways. Thus, whilst the orthoclase oj 
L 2 
