REPORT ON MINERALOGY. 
perties. Both consist of bisilicates of a mixed base, the in¬ 
gredients of the mixture being lime, magnesia, protoxide of 
iron and of manganese ; but in the case of pyroxene the lime 
is equal to the sum of all the other ingredients of the base; 
in diallage the magnesia takes this predominant place; and 
in hypersthene, or bronzite, an intermediate composition 
prevails. / 
Nor does it appear that the optical properties of minerals, 
valuable as they are, can help us out of this uncertainty. It 
may hereafter be found that the distinctions marked by such 
properties are more clear and constant than any others. But this 
is at present uncertain, and there exists a possibility, as yet not 
disproved, that a very minute change of composition may affect 
the optical properties much more than it affects any others. 
However this be, it is clear that the optical distinctions cannot 
take the place of the familiar divisions which the mineralogist is 
accustomed to use. The kinds of Apophyllite which have 
been termed Tesselite and Leucocyclite, in consequence of 
the curious optical phenomena which they exhibited to Sir 
D. Brewster and to Sir J. Herscliel, do not appear to be di¬ 
stinguishable by the eye, or by the common tests of chemistry, 
from other kinds of the same mineral. Sir J. Herschel found in 
a single fragment of a crystal of this substance, three portions, 
each possessing distinct and peculiar properties. Sir David 
Brewster’s tesselites had, in like manner, a difference of optical 
structure in different points. It is clear that species, discrimi¬ 
nated by such differences, cannot easily be employed in classi¬ 
fying mineral specimens ; and it is at present difficult to foresee 
the place which these differences, when they are more fully 
known, will occupy in our mineral systems. 
The angle between the axes of topaz from different localities, 
is also said to vary very considerably. But one of the kinds of 
minerals in which this perplexity appears to be greatest, is the 
micas. Substances which have been referred to this species 
(or group, of whatever kind it be,) have been found ( Bern! . ii. 
149,) to have, some of them a single attractive, some a single 
repulsive axis : others (the more common kinds,) have two axes, 
and the angle between the resulting optical axes has been found 
to vary from 50° to 76°; and though the chemical analyses of 
these different kinds of mica have given results sufficiently 
variable, it does not appear that any steady connexion between 
the composition and the optical properties has yet been dis¬ 
covered. Sir D. Brewster ( Journal , 1825, ii. 205,) has shown 
that a particular kind of mica which he examined (lithion mica), 
contained a combination of both uniaxal and biaxal crystals. 
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