108 J.J. Harris Teall—Cheviot Andesites and Porphyrites. 
Note.—My friend Dr. Trechmann, of Hartlepool, has forwarded 
specimens of the rocks referred to in this paper to Prof. Rosenbusch, 
of Heidelberg, who is now working upon them. Hearing that Prof. 
Rosenbusch had not identified a rhombic pyroxene in the pitchstone 
porphyrite, I ventured to write to him asking for information on 
this point, and stating that I should be glad to add a note on the 
subject to the above paper, which was then in the hands of the Editor. 
He has very kindly acceded to my request. After stating that 
there is undoubtedly a rhombic pyroxene, probably hypersthene, 
as well as augite, in the rock, and that his researches led him to the 
conclusion that the former is the predominating bisilicate constituent, 
he says :—‘‘ You may distinguish the two nearly related minerals by 
the following characters. The monoclinic augite is not in the least 
pleochroic; green in all sections. The ortho-rhombic pyroxene is 
strongly pleochroic in sections which are not too thin: the oscillations 
parallel to the vertical axis are green, those parallel to the horizontal 
axes are red and yellow respectively. The plane of the optic axes 
lies parallel to the green and red rays; that is to say, in the plane 
of perfect pinakoidal cleavage.” Speaking of the affinities of this 
rock he says :—‘‘If you should find that in general the monoclinic 
pyroxene is the dominating bisilicate, then the Cheviot porphyrite 
would be very closely related to the Permian palatinites of the 
Nape Valley; but if, as I must conclude from my observations, 
the ortho-rhombic pyroxene is the principal bisilicate constituent, 
then the rock would be the porphyritic equivalent of the ancient 
norites and directly equivalent to the recent hypersthene-andesites.” 
We shall await with great interest the publication of Professor 
Rosenbusch’s work on this remarkable rock. It seems clear that 
some of the pyroxene which I had taken for augite must be 
regarded as hypersthene or some allied mineral. M. Fouqué 
describes hypersthene as occurring in the Santorin rock, and on 
re-examining my Santorin slides, ] am quite unable to distinguish 
between the pyroxenic constituents of these and of many of the 
Cheviot rocks. Sections across the prism show that corresponding 
faces are similarly developed, that the cleavages are similar, and 
that twinning parallel to a pinakoidal face also occurs in both rocks. 
Pleochroism is also present in both cases. If augite predominates, 
then the Cheviot rock will be an augite-andesite; if hypersthene, 
a hypersthene-andesite. A hypersthene-andesite from St. Egidi, in 
Steiermark, has been described by Niedzwiedzki (Min. Mitt. 1872, 
vol. iv. p. 258), and it is worthy of note that the bulk-analysis of 
the Cheviot-andesite agrees more closely with this than with the 
Santorin rock, especially as regards the presence of water in the 
base. Thus the St. Egidi rock contains: Silica 61°37, Alumina 
15°76, Ferrous oxide 2:94, Ferric oxide 4:06, Lime 7:27, Magnesia 
2°86, Soda 3:04, Potash 0:71, Water 2:64. Specific gravity 2°72. 
