J. J. Harris Teall—Cheviot Andesites and Porphyrites. 255 
The microscopic ground-mass consists of an ultimate base of clear 
isotropic glass, which is thickly crowded with opaque and reddish- 
brown granules, minute crystalline grains of pyroxene, and in places 
with the mica above described. 
Mica PoRPHYRITES. 
These rocks are reddish-purple, and sometimes even brick-red in 
colour. The ground-mass is compact, and contains more or less 
altered felspars, usually of the same colour as the matrix, and either 
biotite or its chloritic alteration product. 
Red porphyrite, Alwin, one-third of a mile below Kidlandlee Dene.— 
This rock is exposed on the left bank of the Alwin for a distance of 
about 24 yards, and it appears to form a dyke intersecting the ordinary 
porphyrites. Specific gravity 2°52. The porphyritic felspars and the 
ground-mass are now brick-red in colour, and the mica has lost its 
lustre, but in many cases retains its characteristic form. Under the 
microscope the felspars are seen to have the turbid aspect of the ortho- 
clase of granites and to contain specks and patches of ferrite. They 
have lost to a great extent their original homogeneous character, but 
still retain a sufficient amount of individuality to show that they are 
closely striated, and that the extinction positions of adjacent lamellee 
in sections taken at right angles to the twinning plane make ex- 
tremely small angles with each other (? oligoclase). Apatite prisms 
occur as inclusions in the felspar. The altered biotite is extremely 
characteristic; thus in the sections more or less at right angles to 
the vertical axis narrow, irregular, alternating bands and patches of 
chloritic substance, brown comparatively unaltered mica, and opaque 
iron oxides may be observed. The chloritic bands usually have a wavy 
aspect. ‘The iron oxides are partly in the condition of ferrite, partly 
in that of opacite. 
The ground-mass appears a bright- red colour by reflected light, 
owing to the abundance of ferrite. Its true nature is very difficult 
to make out. Quartz is certainly present in considerable abundance, 
and is probably to a great extent, if not entirely, of secondary origin. 
Scattered specks of opacite also occur. 
Red Porphyrite, Biddleston, near Alwinton. —This rock is exposed 
in a quarry about 100 yards from the road and on the north side, 
near the final e of the Biddlestone of Biddlestone Hall, in Ordnance 
Map 108 N.E. It is very similar to the rock just described, but the 
mica is in a fresher condition and shows the characteristic lustre. 
This may be simply owing to the fact that as this rock is quarried, 
one is able to get better preserved specimens. The microscopic 
characters do not require any, special description. 
Purple Micaceous Porphyrite, Skirlnaked, near Wooler.—This rock 
is exposed close to a bridge near the point where the Carey Burn 
joins the Harthope Burn. My attention was called to it by Mr. 
_J. Watson, of North Shields. J think it forms a dyke in the por- 
phyrites of that district, but I cannot speak positively on this point. 
The ground-mass is compact and is of a reddish-purple colour; it 
contains pink felspars and a considerable amount of biotite and its 
