THE ROCKS OF PLYMOUTH. 225 
help to explain the difficulty of distinguishing the real character 
of so many of our schistose rocks, Calc-schist occurs at St. 
Budeaux, and veins of red jasper among the Compton lavas. 
Brent Tor.—But we cannot pass over Brent Tor, the presumed: 
parent, in part at least, of the series. | 
On a casual view Brent Tor appears to be mainly composed of 
a dark-red vesicular rock, at once identifiable as a pumice, some of 
the cavities of which are filled with (for the most part) siliceous 
matter; and of a more compact, ashy-looking rock, of a dark- 
reddish-grey : but closer investigation shows that there are several 
other varieties. Mr. Rutley, in his work on “Brent Tor,” 
describes no fewer than seven sections—pumice-breccia, devitrified- 
rhyolite, scoriaceous-lava, decomposed basalt-lava, and basalt. 
These rocks really form a sub-series to themselves, though, as 
already noted, the ashes, &c., of the vicinity very much resemble, 
and in some cases are identical in texture with, the schistose 
ashes and amygdaloidal rocks of our own immediate neighbour- 
hood. 
I have examined six sections of the Brent Tor rocks, some of 
which agree with Mr. Rutley’s descriptions, and others vary. 
The “ pumice-breccia” section contains in the matrix a number of 
minute doubly-refracting microliths and granules, with calcite 
here and there. The pumice is seen in vesicular patches. The 
“rhyolitic-breccia” has a devitrified felsitic magma, which shows 
fluxion structure, and contains fragments of vesicular rock. Quartz 
occurs in granules, strings, &c. A finely-vesicular amygdaloid, 
which may be classed with his “‘scoriaceous-lava,” has the cavities 
filled with quartz and calcedony ; the matrix is opaque, with 
abundant microliths. There are a couple of tuffs. One is a warm- 
yellow-drab, brecciated rock, having veins, composed of granules, 
with opaque and ferruginous interstitial matter. The granules 
are chiefly quartz, but some are apparently olivine. Another tuff 

a grey, crystalline, compact rock—is apparently all quartz. A 
fine-grained, dark-purple semi-schistose rock fairly answers the 
description given by Mr. Rutley of an altered basalt, but contains 
scoriaceous matter, the bulk of the slide being opaque and semi- 
vesicular, with microliths developed in the matrix. The whole 
series form a most interesting study. 
