146 «J.J. Harris Teall—Cheviot Andesites and Porphyrites. 
the plane of the section, be examined, it is seen that much more 
than half of them are distinctly dichroic, and that all of these 
extinguish light parallel to the vertical axis. The others are not 
visibly dichroic, and extinction takes place at a very decided angle, 
usually approaching 40° from the vertical axis. If, on the other 
hand, those crystals which are apparently cut at right angles to the 
principal axis (judging from cleavage and outline), are tested, more 
than half of them are found to extinguish light when the diagonals 
of the prism, as indicated by the best developed cleavage-planes, 
coincide with the principal sections of the crossed Nicols.” 
The author describes the pleochroism of the mineral, and remarks 
that it does not seem to differ from that often described for the 
“augite’ of the andesites. The mineral in question was isolated 
by the method of M. Fouqué, and three separate analyses of different 
specimens were made. These agree very well with each other, and the 
following may be taken as representing its composition :—Si0, 51:157 
Al,O;, 2:154, FeO, 18:360, MnO, 0:368, CaO, 3812, MgO, 24-251, 
Total, 100-097. In conclusion, the author states that he does not 
claim all the augite-andesites as hypersthene-bearing rocks, but only 
those with which he is acquainted. 
So far as my own observation has gone, I can confirm the opinions 
of Mr. Whitman Cross, and it will perhaps be as well if I describe the 
rhombic pyroxene as it is now known to me, and indicate the localities 
of the rocks in which J have observed it. Sections more or less parallel 
with the principal axis are usually long, and sometimes very long in 
proportion to their width. The ends of such a section are sometimes 
bounded by two definite crystalline faces, which meet at an obtuse angle. 
Irregular transverse cracks are usually present, and these evidently 
exercise an important influence on the alteration of the mineral. 
Longitudinal cleavage-lines may sometimes be observed, but as a rule 
they are not conspicuous. The dichroism of these sections is well 
marked in sufficiently thick preparations, and the colours vary from 
green to yellowish or reddish-brown; green when the axis les 
parallel with the vibration-plane of the polarizer (short axis of the 
Nicol), yellowish or reddish-brown when the axis lies at right angles 
to the vibration-plane. Cross-sections of the prism show that the 
pinakoidal faces are largely developed at the expense of the faces of 
the prism. The prismatic cleavages are strongly marked, though 
somewhat irregular. The two sets of lines in the cross-section meet 
at an angle of about 90°. The dichroism in these sections is less 
marked than in the longitudinal sections ; the colours being yellowish 
and reddish-brown. The pyroxene in the Cheviot and other ande- 
sites of a similar character occasionally shows twinning parallel with 
a pinakoidal face, as pointed out in my previous paper. I suspect, 
however, that this only occurs in the monoclinic! mineral, though I 
am not able to speak positively on this point. 
1 Mr. Whitman Cross expresses doubt as to the monoclinic character of the pyroxene 
associated with hypersthene in the American rocks. I have not examined a sufficient 
number of sections of the corresponding mineral in the Cheviot rocks to speak 
positively on this point, but I have seen nothing to suggest a doubt as to its being a true 
augite. I therefore speak of it as monoclinic in this paper. 
