J. J. Harris Teall—Cheviot Andesites and Porphyrites. 147 
Putting the above facts together, we may say that the rhombic 
pyroxene, when fully developed, occurs as columnar, doubly termi- 
nated crystals, which are made up mainly of the pinakoidal, and 
only to a slight extent of the prismatic faces. It is markedly pleo- 
chroic, the three axial tints being green, yellow, and reddish-brown. 
The prismatic cleavages are strongly marked. Prof. Rosenbusch 
speaks of a perfect pinakoidal cleavage, but this I have not been 
able to make out in my sections. 
I will now enumerate the rocks in which I have observed this 
mineral, and it must be remembered that in each case there is a 
striking general resemblance to the Cheviot type. In short, there 
is not the slightest doubt that, unless we are prepared to make 
geological age, per se, a point of classificatory value in petrology,’ 
the same name must be applied to each and all of the rocks 
about to be referred to, with the exception of the one from Dorf 
Hladomer. 
The anorthit-trachyte of Szab6? from Lérinezi, West-Matra and 
South Servia, contains this mineral in abundance, and this rock also 
resembles the prevalent Cheviot type in the most striking manner 
in other respects. A hand-specimen from Lérinczi in the British 
Museum may be described as a compact rock with semi-resinous 
lustre and conchoidal fracture. 
Tt also occurs as the characteristic bisilicate in specimens of 
andesite from Kohlbach, Bagonya near Schemnitz, Dorf Nevolut 
near Kremnitz, and Dubnik near Eperies. An andesite-lava from 
a point near the summit of Ararat contains it along with a twinned 
monoclinic and non-dichroic pyroxene, and a similar association 
occurs in the rock which forms the summit of Stromboli. The above 
statements are based on the examination of specimens kindly shown 
to me by Prof. Judd, to whom I am also indebted for the loan of 
Prof. Szabo’s communications. 
As already stated, all the Cheviot andesites contain the mineral in 
a comparatively unaltered condition, while many of the porphyrites 
contain it in an altered form. It occurs also in the so-called augite- 
andesites from Tokaj, Hungary, and in those from Santorin, including 
the lava of 1866. 
A rock from Dorf Hladomer, Hungary, which in other respects 
resembles the hypersthene-bearing andesites, contains as the pre- 
1 T do not wish to be understood as holding the view that there is no difference 
between igneous rocks of different geological epochs. I do hold, however, that a 
petrologist should be able to name a rock without reference to its geological age, 
just as a paleontologist should be able to name a well-preserved fossil without 
reference to its age. If geological age, per se, is to be accepted as a point of classi- 
ficatory value in petrology, then there are, as every field geologist knows, many intrusive 
rocks that we can never name, for we can never determine their geological age. Such, 
for instance, are the rocks forming many of the dykes in the North of England. 
2 Trachyte eingetheilt nach den natiirlichen System. Exposition Universelle, 
Vienne, 1873. 
Classification macrographique des trachytes par J. Szabo. 
Sur la classification et la chronologie des roches éruptives tertiaire de la Hongrie, 
Paris, 1880. Extrait du compte rendu sténographique du Congrés international de 
Geologie tenu a Paris du 29 au 31 aott et du 2 au 4 Sept. 1875. 
