THE ROCKS OF PLYMOUTH. waa 
The general character of the more massive rock is very well 
seen in a section cut from an exposure to the north of Colebrook, 
which shows a greenish-grey mass of felted lath-felspars on a 
ground with a dirty flocculent aspect. There is a good deal of 
magnetite present. No augite is visible, and it has probably been 
decomposed. 
Our fragmental volcanic rocks present some puzzling peculiarities, 
At the Devon end of the Royal Albert Bridge there occurs a rock 
with a schistose-granular and, in part, scaly texture. The micro- 
scope shows it to consist of fragments of broken crystals and lava ; 
but there appears to bea strongly-marked fluxion structure in the 
enclosing network. It is not vitreous, nor does it seem ever to 
have been so. There is no doubt that it is a tuff; but it differs 
from my other examples of this class in the extent and nature 
of the interstitial matter, which suggests to me the active presence 
of highly-heated water, though no doubt pressure has often 
simulated fluidal characters. 
A still more remarkable tuff was found in the course of the 
sewerage works at Compton. It has a scaly, granular texture, 
much as if a number of grains had been interspersed lineally 
between undulating and impersistent slaty lamine. Under the 
microscope the slaty matter is seen to be very fine-grained, with a 
““cobwebby” texture. Such a rock might be formed by the re- 
arrangement under water of granular tufaceous matter and volcanic 
dust, the slaty aspect being given to the finer particles by the 
subsequent pressure, which in the compact, even-textured rocks 
produced slaty cleavage. The scaly parts probably contain a con- 
siderable quantity of volcanic-glass. The granules are chiefly 
felspar and quartz, with a little calcite and iron peroxide. 
A simpler tuff occurs at Crabtree, where a crypto-granular rock 
is seen to enclose distinct patches of slate. This was noted by 
Mr. Prideaux, who, with singular discrimination, pointed out that 
these patches were “formations,” and not “ breccia.” 
Our best defined tuff is, however, again from Compton, and, 
oddly enough, is the example selected for slicing, as best adapted 
to illustrate the most compact form of our lavas. Great was my 
surprise, on examining this slide, to find that it was wholly made 
up of small grains, with a very small quantity of interstitial 
matter. At the other end of this series we have a mere sandy 
aggregate occurring at Egg Buckland—a tuff which has lost its 
