652 J. J. H. TEALL ON THE CHEMICAL AND 
quartz and felspar, a monoclinic and a rhombic pyroxene, and mag= 
netite and ilmenite. 
Rhombie Pyroxene.—This mineral is by no means uniformly pre- 
sent, but where it does occur several sections may usually be recog- 
nized in the same microscopic preparation. . In its most typical 
development it takes on the form of elongated prisms, and shows a 
much more decided approach to crystalline form than the prevailing 
monoclinic pyroxene. Two terminal faces meeting at an obtuse 
angle may sometimes be detected. In the fresh condition this 
mineral appears nearly colourless when examined by ordinary light. 
It develops a fibrous structure parallel with the long axis of the 
prism by alteration, and finally passes over into a green serpentinous 
substance. Examined with a polarizer only, the longitudinal sections 
show a decided pleochroism, appearing colourless or of an extremely 
pale green when the long axis of the prism corresponds with the 
short diagonal of the nicol, and of a pale brown or reddish brown when 
the opposite relation holds. Longitudinal sections generally show 
irregular transverse cracks, and invariably give straight extinction. 
Another and a very characteristic feature is the low order of the 
interference-colours under crossed nicols. In sections in which the 
augite constantly gives the brilliant pinks and greens of the second 
and third orders of Newton’s scale, the rhombic pyroxene gives the 
pale tints of the first order. In a section taken from a Middleton 
specimen I have observed intergrowths of the augite and the rhombic 
mineral. Thus, one crystal of augite cut approximately parallel to 
the clinopinacoid, shows a thin lamella of rhombic pyroxene in its 
centre. The latter mineral gives straight extinction, and polarizes in 
the pale yellow of the first order ; the former mineral extinguishes at 
an angle of 43°, and polarizes in the brilliant green of the third order*. 
In another portion of the same slide two thin lamelle of augite are 
seen to be enclosed in a good-sized erystal of the rhombie pyroxene. 
What name is to be applied to this mineral? In the absence of 
more direct optical or chemical evidence the properties of colour 
and pleochroism are the only ones available for the purpose of 
distinguishing between enstatite, bronzite, and hypersthene. Typical 
enstatite appears devoid of pleochroism. LBronzite is slightly and 
hypersthene markedly pleochroic in thin sections. In other words, 
the depth of colour and the intensity of pleochroism inerease with 
the amountof iron. Now the pleochroism of the mineral in question 
is decided, though much less marked than that of the hypersthene 
of the andesites, and therefore the term bronzite seems to be the 
most applicable. It must be admitted, however, that we want a 
general term for the rhombic pyroxenes which are now being recog- 
nized in so many rocks, and which are incapable of precise deter- 
mination. Prof. Rosenbusch uses the term “ Enstatite” to cover 
the entire group. Prof. Tschermak + and Dr. F. Becke ¢ on the 
* The quartz wedge described by Sorby in the ‘Monthly Microscopical 
Journal, 1877, p. 209, was used for these determinations. 
t Lehrbuch der Mineralogie, 1854, p. 4386. 
t “ Ueber die Unterscheidung von Augit und Bronzit in Dimnsehliffen.” Min. 
Mitth. Bandy. 1883, p. 527. 
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