BY THE REV. J. MILNE CUBRAH. 211 



of a deep brown colour, and sometimes of a rich sap green. Nearly 



every slice has a favourable section on which the angle, formed by 

 an axis of elasticity and a crystallographic axis, can be measured. 

 Sometimes it is not easy to distinguish between hornblende and 

 biotite in slices of the Bathurst granite. For the information of 

 students who may make use of these notes, I may just indicate the 

 difference. The micas, including of course biotite, show no sensible 

 dichroism in sections parallel to the base. In sections across the 

 cleavage the biotites will show very strong dichroism on rotating 

 the lower Nicol prism. Hornblende is also dichroic, but a few 

 sections can usually be found on the slice parallel, or nearly 

 parallel, to the base ; these will show two sets of cleavage. 

 Sections of hornblende, parallel to the vertical axis, show but one 

 set of cleavage lines, and in this resemble mica. But the cleavage 

 planes of the hornblende are generally coarse, or seldom so close 

 as those in mica. Mica, too, has usually a more ragged look than 

 hornblende, and the ends of the lamina? have a frayed-out appear- 

 ance. Finally, unless the section be cut exactly parallel to the 

 orthopinacoid, hornblende does not extinguish when the cleavage 

 lines are parallel to a diagonal of the Nicols. Between 

 crossed Nicol prisms all sections of biotite will be black when the 

 cleavage corresponds with the plane of vibration of either Nicol, since 

 the cleavage corresponds with an axis of elasticity. With horn- 

 blende this is not the case, and, in the larger number of its sections, 

 the point of maximum darkness will be obtained when the cleavage 

 makes a certain, though not great, angle with the plane of the light. 



Biotite. — Biotite is common in all the slices. It appears as 

 irregular plates, with parallel striae, corresponding to the cleavage 

 on sections, parallel to the vertical axis. The absorption exhibited 

 by rotating the polarizing prism under the section is very marke J. 

 Sometimes flakes show of a light brown colour, without any 

 cleavage lines, exhibiting no dichroism. These I take to be 

 biotites cut parallel to the basal planes or cleavage. They 

 resemble, in every respect, flakes or plates cleaved from biotites 

 and mounted separately for comparison. 



