THE ROCKS OF PLYMOUTH. TAL 
Apart from the vesicular varieties—and I include under this 
head both the forms in which the cavities are empty, and those in 
which they are filled with various minerals, and therefore become 
amygdaloidal—our Devonian lavas present many difficulties of 
identification to the naked eye. It is frequently the case that, 
within a few yards of an unquestionable lava-flow, we find a rock 
in no way distinguishable from ordinary clay-slate, and between 
the two such a gradation that it is not possible to say with 
precision where the igneous rocks end and the sedimentary 
begin. 
This is due to two leading causes. In the first place, the 
volcanic rocks, being contemporaneous with the rocks among 
which they are found, have been subjected to precisely the same 
subsequent external conditions of change. In the second, they 
consist not only of lavas, but of fragmentary materials—ashes and 
tuffs, the latter differing from the former in the fact that while 
the ashes are mainly of aerial deposition, the tuffs have been formed 
under aqueous influences—the disintegration of the parent material 
in the one case being mechanical, in the other being caused by the 
action of water upon molten lava, which sudden cooling has 
shattered into fragments. Now if no change had taken place in 
these rocks since they were ejected or deposited, they would 
be easy of identification, though the tuffs might be confounded 
with sandstones, as indeed has been the case with certain of 
their more friable varieties. But there has been at least one 
very great change, in the development of slaty cleavage by the re- 
arrangement of particles under pressure, and as the ashes and 
tuffs have in their degree been amenable to this action, they too 
(and occasionally also the lavas) have acquired the fissile character 
which is at times so puzzling. But the microscope sets nearly all 
doubts at rest. . 
Making allowance for the changes superinduced since the 
formation of our local volcanic series, these present precisely the 
same phenomena as lavas and ashes of the present day. True the 
vitreous character is not, as a rule, distinct, but it is by no means 
wholly lost. It is seen, for example, clearly in the Brent Tor 
pumice, and is well marked in a singular slaggy rock at Egg 
Buckland. This rock has the casual aspect’ of an amygdaloid, 
containing rounded white fragments in a dull, compact, semi- 
glassy base. Microscopic examination, however, indicates that it 
