254 J. Jd. Harris Teall—Cheviot Andesites and Porphyrites. 
Valley.—This is an interesting rock, to which my attention was first 
called by my friend, Mr. James Watson, of North Shields. Macro- 
scopically it consists of porphyritic felspars, measuring from two to 
three mm. across, imbedded in a very dark compact matrix. The 
specific gravity is 2°74, and the silica per-centage slightly over 60. 
Under the microscope it is seen to contain mica, a colourless pyroxene, 
magnetite, and apatite, in addition to the felspar. The ultimate base 
of the rock is a clear isotropic glass, but this is thickly crowded with 
minute granules, some of which appear opaque, while others have a 
deep reddish-brown colour. : 
The Felspars. —These occur for the most part in the form of more 
or less broken crystals. Both large and small have the same general 
character, and I am therefore inclined to refer them all to one and 
the same stage of rock consolidation. 
They are frequently honeycombed with irregular inclusions of 
the ground-mass, but their most striking peculiarity is a remarkable 
cloudy appearance which can be shown, by means of a magnifying 
power of 1000 diameters, to be due to minute inclusions and not to 
alteration. The sections at right angles to the twinning plane are 
seen also under ordinary light to be very finely striated in conse- 
quence of the arrangement of the extremely minute inclusions in 
lines, or rather planes parallel with the brachypinakoid. Under 
polarized light, the same sections show the characteristic banding 
of plagioclase, and the maximum angle which has been observed 
between the extinction positions of adjacent lamelle is 54° (maxi- 
mum for oligoclase 37°). The number of sections examined is, 
however, too small to warrant a definite conclusion. 
The Pyroxene.— This occurs in good-sized crystalline grains, 
having the cleavages and extinction angles of augite (monoclinic) 
and also as minute crystalline granules in what may be called the 
microscopic ground-mass. The larger grains rarely show any traces 
.of definite geometric form, but they are frequently twinned. The 
absence of colour appears to suggest that we are here dealing with 
the variety of augite known as salit. 
I may mention in passing that this pyroxene occurs in almost all 
the quartz-bearing holocrystalline rocks of the Cheviot district; at 
least in almost all those which have passed under my notice. Prof. 
Rosenbusch calls this rock an augite-porphyrite. 
Mica.—A mineral having the dichroism of biotite, and therefore 
presumably to be referred to that species, occurs in extremely minute 
crystals and crystalline particles. ‘The dimensions of an average 
section taken at right angles to the vertical axis are ‘002 X:007 mm. 
In some portions of the ground-mass this mineral occurs in great 
profusion, two or more crystals lying over each other in the thickness 
of the preparation, whilst in other portions it is entirely absent. 
Apatite occurs in abundance. It is clear and colourless, and an 
average longitudinal section measures ‘07 X°25 mm. 
Magnetite is present in the form of irregular grains of considerable 
size and also as minute granules in the ground-mass. It is, however, 
not very abundant. 
