258 J.J. Harris Teall—Cheviot Andesites and Porphyrites. 
Very coarse breccias containing angular fragments of the normal 
porphyrite, measuring many inches in diameter, occur at a point in 
the Coquet, about a quarter of a mile above Windy Haugh, and again 
at Blindburn. At the former locality the matrix is fine-grained and 
green in colour. It iscomposed largely of quartz grains and a chloritic 
mineral which appears, in part at any rate, to arise from the altera- 
tion of mica. A little felspar is also present. The included frag- 
ments of porphyrite contain veins of quartz and chalcedony, which 
are not continued into the matrix, thus showing that these veins 
were produced in the porphyrite before it was broken up to form the 
breccia. A portion of the green matrix yielded 77:4 per cent. of 
silica. At present I am not able to give a satisfactory account of the 
origin of these coarse breccias. 
TUEDIAN ‘ PORPHYRITES.” 
My knowledge of the igneous rocks of Tuedian (Lower Carboni- 
ferous) age, which occur in the neighbourhood of the Cheviot 
district, is solely derived from the examination of a specimen kindly 
given to me by Mr. Clough. It is labelled bedded-porphyrite, 
Stichill. Macroscopically it consists of a compact dark-purplish 
ground-mass, with reddish patches and blotches, throughout which 
are scattered large glassy felspars, measuring sometimes 8 or 4 mm. 
across, and a few black crystals of pyroxene. The specific gravity 
of the rock is about 2-95. Its microscopic structure is represented 
in Plate VI. 
The rock is holocrystalline, and the original constituents, mention- 
ing them in the order of their probable formation, are apatite, mag- 
netite, olivine, anorthite or bytownite, felspars of the ground-mass 
(? labradorite), and pyroxene. The secondary products are serpen- 
tine and related minerals and iron oxides. They result mainly from 
the alteration of olivine. Apatite occurs in long acicular prisms. 
Magnetite in crystals and grains of tolerably uniform size. It is 
abundantly and evenly scattered through the rock, and its relation 
to the other constituents is well shown in the Plate. The olivine 
was present originally both in the form of good crystals with definite 
boundaries and as grains. It is now represented by the usual — 
serpentinous pseudomorphs containing irregular cracks filled with 
iron oxides. Kernels of the unaltered mineral may frequently be 
observed. Three good sized olivines are shown in the upper figure, 
and in two of these unaltered portions of the original mineral may be 
recognized by the lighter shade. 
The large porphyritic felspars occur both as fairly good crystals 
and as fragments. They show the usual polysynthetic twinning, and 
are comparatively free from inclusions. Now and then magnetite, 
and more rarely small augite granules, occur as inclusions. The 
latter, however, when present, are near the margin of the felspar, at 
least in the few cases I have observed, and I am inclined to explain 
this by supposing that the marginal zone of the felspar substance 
has been added during a later stage in the process of rock consolida- 
tion than that in which the main growth of the crystal took place, 
