THE ROCKS OF PLYMOUTH. 247 
axinite, biotite, calcedony, calcite, chiastolite, chlorite, diallage, 
dolomite, enstatite, epidote, felspar (orthoclase and plagioclase), 
fluor, garnet, gilbertite, hematite, hornblende, ilmenite, indicolite, 
jasper, kaolin, lithomarge, limonite, magnetite, manganese, mus- 
covite, olivine, opal, prase, pyrites, quartz, serpentine, sericite, 
schorl (tourmaline), steatite, sphene, zeolite, zircon. 
In order to explain the structure of the rocks of the locality 
more clearly than by mere description, I have prepared a plate 
‘containing sketches of characteristic sections of twenty-four 
of the most typical varieties. In several cases the structure 
comes out clearly only under the polariscope, and from the 
absence of colour and the necessary smallness of the drawings, it 
is impossible to be absolutely precise in all points of detail. Still 
the sketches, which have been drawn direct from the actual 
slides, very fairly illustrate their general features, and will 
probably be found of service in elucidating this paper. A 
description of each section follows under its appropriate letter. 
With two or three exceptions a one-inch power was used. 
A. Lava, near Colebrook.—Greenish-grey, fine-grained rock, 
soft, with ochreous spots, and cavities with ochreous lining. © 
Microscopically it shows a greenish grey-brown partially felted 
mass of lath-shaped felspar crystals (plagioclase), with crystals of 
magnetite and pyrites. The ground has a dirty flocculent look. 
B. Amygdaloidal-Lava, St. Andrew Churchyard.—Light-grey 
compact rock at junction with slate. The matrix is much like the 
Colebrook rock, but finer-grained. Vesicles filled with calcite. 
These characters are generally those of our amygdaloidal rocks, the 
originally vitreous character of which is more or less apparent. 
The Honicknowle variety, in addition to pyrites and magnetite, 
contains ilmenite. 
C. Amygdaloid, Whifferton.— Matrix olivaceous-green, with 
kernels of pink calcite. Chloritic ground between kernels broken 
by dark strings and patches, giving a flowing texture, and contain- 
ing partially nucleated subvesicles. Much-altered lava. 
D. Felstone, near Camel’s Head Bridge, Saltash Road.—Warm- 
ochreous-drab in colour, soft, with banded and veined aspect. 
Under the microscope it shows a grey-speckled, granular-glassy, 
patched texture. Probably an ash much altered. 
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