J. J. Harris Teall—Cheviot Andesites and Porphyrites. 161 
Silica 64:2 
Alumina ax 16:0 
Tron oxide! ... 4:3 
Lime jb7i 
Magnesia 2°5 
Soda 2°9 
Potash 5°9 
Loss 38 
100°8 
On comparing this analysis with the one given in the last paper, 
it will be seen that the lime is replaced by alkalies, and that the 
proportion of soda to potash is reversed. Notwithstanding this, the 
porphyritic felspars appear to be entirely plagioclase. 
Microscopically the rock is a very interesting one. The large 
felspars are fresh and of the usual character, so that a special descrip- 
tion of them is unnecessary. The pyroxene is in the condition of 
a green slightly dichroic mineral, sometimes showing definite optical 
characters and at others passing into the condition of serpentine. 
A fibrous structure may sometimes be observed in it. Alteration 
appears to have proceeded along the cracks, as in the case of olivine, 
and sometimes iron oxides have been formed along these cracks. 
These characters connect this mineral with bastite, the well-known 
alteration product of enstatite. Magnetite occurs as large crystals 
and grains and also as minute grains abundantly scattered throughout 
the ground-mass together with ferrite. Apatite prisms are not 
uncommon. The iron oxides also have segregated in places in 
a patchy manner. The ground-mass itself is mainly composed of 
minute felspar microlites intimately interwoven with each other 
(mikrolithenfilz), and I do not think, though on this point it is 
difficult to be positive, that any portion of it remains persistently 
dark between crossed Nicols. 
Coquet, one mile below Windy Haugh.—A dark purplish-grey com- 
pact ground-mass containing porphyritic felspars and a few elongated 
amygdaloids all lying with their longer axes parallel to one another. 
The form of these amygdaloids is probably due to the flow of the 
semi-fluid magma in which the original vesicles were produced. 
The amygdules consist of quartz and calcite. Specific gravity of the 
rock, 2°63. Under the microscope the ground-mass is resolved into 
a felted aggregate of extremely minute and somewhat ill-defined 
felspar microlites and specks of opacite (? magnetite). It is difficult 
to say whether any true isotropic substance remains. The porphy- 
ritic felspars contain the usual inclusions and exhibit the character- 
istic twin striation. There is no pyroxene at present existing in the 
single section of this rock which I have examined, but there are 
numerous patches of calcite and quartz, and some of the outlines of 
these patches suggest the form of pyroxene. Pseudomorphs after 
pyroxene of this kind are not however seen in other specimens of 
the Cheviot rock which I have examined. 
Coquet, quarter of a mile above Windy Haugh.—This rock is 
exposed only a short distance from the glassy andesites described in 
the previous paper. It consists of a dull compact ground-mass, 
1 Estimated as Fez O03. 
