150 
THE CAMBRIAN VOLCANOES 
BOOK III 
a grouiidinass remaining dark between crossed nicols, Imt with luminous 
points and filaments, and an occasional spherulite giving the usual cross in 
polarized light. Lapilli of an older tuff may here and there he detected. 
A few angular and subangular grains of quartz are scattered through 
the rock. The lapilli are bound together by a fiuely-granular dirty-green 
substance. 
As a typical illustration of the minute structure of the felsitic tuffs, 
I may refer to the rock No. V. of the foregoing analyses. It is composed 
mainly of fragments of various felsites, many of which show good flow- 
structure. Large, and usually broken, crystals of orthoclase are dispersed 
among the other ingredients. Here and there a fragment of diabase may 
be detected ; but I could find no trace of pieces of the peculiar micro- 
crystalline spherulitie quartz-porphyries of St. David’s. There is but little 
that could be called matrix cementing the lapilli together. The presence 
of fragments of diabase may possibly reduce the proportion of silica and 
increase that of magnesia, as compared with what would otherwise have 
been present in the rock. 
Some of the tuffs appear to have been a kind of volcanic mud. A 
specimen of this nature collected from the road-side section, north of the 
Board School, presents a finely-granular paste enclosing abundant angular 
and subangidar lapilli of diabase, a smaller proportion of felsite (sometimes 
displaying perfect flow-structure), broken plagioclase crystals, and a greenish 
micaceous mineral which has been subsequently developed out of the matrix 
lietween the lapilli. 
Though they lie in the sedimentary series above the main volcanic 
group, I may refer here to certain thin bands of tuff at Castell, on account 
of their interest in relation to the true Cambrian age of the volcanic group. 
They are not quite so fresh as the tuff that occurs in thicker masses, but 
their volcanic origin is readily observable. One band appears to be made 
up of the debris of some basic rock, like the diabase of the district, through 
which detached plagioclase crystals are scattered. The lapilli are subangu- 
lar ; and aromid their border a granular deposit of haematite has taken 
place, giving a red colour to the rock. Another band presents small angular 
lapilli, almost entirely composed of a substance which to the naked eye, or 
with a lens, is dull, white and clay-like, easily scratched, and slightly unctuous 
to the touch. Under the microscope, with a low power, it becomes pale 
greyish-green and transparent, and is seen to consist in large part of altered 
felspar crystals, partially kaolinized and partially changed into white mica 
and calcite. These scattered crystals are true volcanic lapilli, and have not 
been derived from the meclianical waste of any pre-existing volcanic rock. 
In the tuffs interstratified with the conglomerate, at the quarry above 
Bortli-elais, though much decomposed, crystals of plagioclase can likewise 
still be traced. These strata are also true tuffs, and not mere detritus due 
to mechanical degradation (see Tig. 41). 
The genei’al result of the study of the microscopic structure of the 
Cambrian tuffs of St. David’s may be briefly summed up as follows : — 
