224 JOURNAL OF THE PLYMOUTH INSTITUTION. 
coherence, and which probably few would imagine to be a volcanic 
rock at all. Indeed, the Crabtree tuff is at times very deceptively 
like sandstone. 
The most characteristic ash is perhaps that found at Mount 
Batten, a soft semi-schistose rock, with ochreous patches. Under 
the microscope it has a very irregular look, and shows no signs 
of crystalline structure beyond the presence of some grains of 
magnetite. It is clearly made up of very fine particles of volcanic 
matter, and owes much of its appearance to the oxidation of its 
iron,! 
A very interesting ash also occurs at Radford, and in its colour 
is. no doubt the origin of that name. It is a dark-red, fine- 
grained, open-textured rock, which has some rather puzzling 
characteristics. For one thing, it is neither vesicular nor scoriaceous, 
and its openness appears therefore difficult to account for, unless 
some of its constituents have been removed. <A few fine calce- 
donic veins are apparent. The blowpipe shows that the colouring 
matter is iron, and that silica is largely present; and on treating 
some of the fragments with warm hydrochloric acid, the 
broken felspar granules, which form the chief constituent, are 
revealed. 
A noteworthy felstone, not improbably an altered lava, occurs 
in the Saltash Road, near Camel’s Head Bridge. 
Various names have been given to our local lavas, and there is 
a good deal of what may be regarded as an intermediate character 
about them. They. have been termed diabases, andesites, and 
basalts; but probably they range between the two latter, which 
indeed have points of close relationship. The schistose-tuffs have 
been associated with the Continental schalstein. 
Two of the most interesting rocks connected with our volcanic 
series are a slate from St. Budeaux and another from Drake’s Island. 
Their interest consists in this, that while they appear true slates 
in general composition and structure, they contain little patches, 
These the microscope reveals to be fragments of consolidated lava, 
which fell into the bed of the sea wherein the silt was being 
deposited that, in the course of ages, has become clay-slate. 
The fragments are really volcanic fossils, and their occurrence will 
1 The ash beds not infrequently contain fossils. Encrinital plates occur in 
the Batten ash; corals, &c., in ash near Dartington ; and Phacops levis in 
ash at Highweek. 
