104. J.J. Harris Teall—Cheviot Andesites and Porphyrites. 
rhombic pyroxene, viz. augite. Many of the lath-shaped sections are 
without well-marked longitudinal cleavage-cracks and a large num- 
ber of them certainly extinguish with their edges parallel to the 
vibration plane of either the polariser or analyser. It is possible 
of course that the pyroxenic constituents of the rock may comprise 
more than one species of mineral. The pyroxene forms a very sub- 
ordinate part of the whole mass. It is on the whole remarkably 
fresh, although occasionally it may be seen altered into a yellowish- 
brown substance. Inclusions are much less common in the augite 
than in the felspar, and a zonal banding is entirely wanting. When 
inclusions occur, they consist either of glass cavities with fixed 
bubbles,! or of colourless microlites. The pyroxene is distributed 
irregularly through the ground-mass of the rock, just as in the 
Santorin augite-andesites, several grains frequently occurring near 
together, and sometimes interfering with each other. 
The constituents which remain to be described make up the ground- 
mass of the rock. 
Felspars of the ground-mass.—These may appear in lath-shaped 
sections measuring as much as ‘14mm. by ‘4 mm., or as extremely 
minute microlites. The latter sometimes occur in countless numbers 
and almost to the exclusion of the former; thus producing a ground- 
mass which may be described as a felted aggregation of microlites in 
a glassy base (mikrolithenfilz). This type of ground-mass occurs 
abundantly in the augite-andesites of Santorin. Two of my slides, 
one labelled Aphroessa 1866, and the other Palzea Cap., show it to 
perfection, and bear the most striking resemblance to some of the 
Cheviot andesites. Fluidal structure is admirably shown in both 
the Santorin and Cheviot rocks. It may be as well to mention here 
that the Santorin® andesites are described by Zirkel as having a 
pitchstone-like aspect.? In the majority of the Cheviot andesites 
examined by me the fluidal structure is absent, and the felspars of 
the ground-mass vary considerably in size. The sections of the 
felspars are short and lath-shaped, often with ragged or bifid termi- 
nations. They are without inclusions, and therefore differ markedly 
from the smaller porphyritic crystals. Under crossed Nicols they 
mostly present the characters of simple individuals or binary twins; 
but, as is now well known from the researches of M. Fouqué* on 
the Santorin lavas, we must not infer from this that they are sani- 
dines. I have made attempts to determine both the large and small 
felspars by optical methods applied to the rock-sections; but the 
results are so unsatisfactory, that I prefer, at any rate for the present, 
to leave the question entirely open. No doubt the prevailing felspar 
is plagioclase, and it is of course highly probable that the felspars of 
1 T assume they are glass cavities, because I have not seen any spontaneous move - 
ment of the bubbles even in the smallest of them. 
2 Zirkel, F., Ueber die mikroskopische Zusammensetzung der diessjahrigen Laven 
von Nea Kammeni, Neues Jahrbuch, 1876, p. 769. Also Mik. Besch. p. 390. 
3 The same author describes the augite-andesites of the 40th parallel of North 
America as having in most cases a resinous lustre. Microscopical Petrography of the 
40th Parallel, Washington, 1876, p. 221. 
4 Santorin et ses Eruptions, 1579, Paris. 
