256 J.J. Harris Teall—Cheviot Andesites and Porphyrites. 
chloritic alteration product. Under the microscope, the porphyritic 
felspars are seen to be plagioclase, at any rate for the most part, and 
they give very low extinction angles in the zone of the macro-diagonal 
axis. The biotite is frequently very fresh, and in sections more or less 
parallel with the principal axis varies in tint from a deep rich brown 
to a pale yellow, as the polarizer is rotated. In the rock last described 
the mica occurs as large flat plates, but in the one now under con- 
sideration it occurs as short stumpy prisms. Various stages of altera- 
tion may be traced in the thin section; thus some crystals have been 
attacked at their edges, others have been altered along definite bands, 
and some are in the condition described as characteristic of the mica 
in the specimen of red porphyrite from the Alwin. 
The other distinctly recognizable constituents are quartz, a mineral 
of pyroxenic aspect, magnetite or ilmenite and apatite; the last 
mentioned occurring as inclusions in all the other definitely crystal- 
line constituents, and also in the ground-mass. 
Under a high power the ground-mass is seen to be thickly charged 
with extremely minute indistinct granules and fibres, together with 
larger irregular patches due to collections of ferrite granules. With 
crossed Nicols it presents the appearance of a confused mass of 
irregular crystalline particles, in which quartz may be distinctly 
recognized. Much of this quartz is doubtless secondary. 
A rock of precisely similar character to the one above described 
occurs on the §.W. face of Goldsmouth Hill, near Yetholm, where 
it also probably forms a dyke. 
An allied rock also occurs one mile below Langlee Ford, in the 
Harthope Valley. The mica, however, is not so abundant here, and 
the micro-crystalline character of the ground-mass is much more 
pronounced, and in many places puts on the aspect of micro- 
pegmatite. 
Reddish-purple Porphyrite, one mile up Allerhope Burn and a quarter of 
a mile below Kidlandlee Dene, in the Alwin.—The specimens from these 
two localities are, for purposes of description, identical. The ground- 
mass is compact, and gives its colour to the rocks. Throughout it 
are scattered lighter felspars and dark-green patches of somewhat 
irregular shape. Under the microscope the felspars are seen to be 
saussuritic, but they retain sufficient individual action on polarized 
light to show that they are twinned on the plagioclase type, and that 
the extinction angles are low. The green mineral gives aggregate 
polarization, and is frequently associated with quartz. Calcite also 
occurs, and is usually inter-crystallized with quartz. The ground- 
mass is micro-crystalline, and together with felspars is charged 
with ferrite. A few parallel bands of inclusions occur near the 
edges of the felspar sections. Apatite crystals and opacite grains 
occur sparingly. It is difficult ‘to say what the original ferro- 
magnesian mineral was, but I am inclined to regard "it as mica. 
Specific gravity of Allerhope specimen 2°59. 
Reddish-purple Porphyrite, the Alwin, 100 yards above Allerhope 
Burn.—This is almost certainly a true lava, as it shows well-marked 
finidal structure in the thin sections. Macroscopically the rock is 
