CHAP. X 
ST. DA VID’S 
151 
1. These pyroclastic deposits are almost wholly composed of fragments 
of eruptive rochs, sometimes rounded, but usually angular or suhangular. 
In the more granular varieties very little matrix is present ; it consists of 
fine debris of the same materials. No detached microlites have been noted, 
such as are common among modern volcanic ashes ; hut there are abundant 
ejected crystals. In these respects the Cambrian tuffs resemble those of 
the other Paleozoic systems. A mingling of grains of quartz-sand may 
indicate the intermixture of ordinary with volcanic sediment. 
2. They may be divided into two groups — one composed mainly of 
fragments of diabase or other similar basic rocks, the other of felsite. The 
former group has doubtless been derived from the explosion of such rocks 
as the diabase -sheets of the district. The felsitic tuffs have not been 
observed to contain any fragments of the microcrystalline quartz-porphyries 
of St. David’s. They have been derived from true fine-grained felsites or 
rhyolites. There are various intermediate varieties of tuff, due to the 
mingling in various proportions of the two kinds of debris. 
3. They are marked by tlie presence of some characteristic features 
of the volcanic vents of later Palseozoic time, and in particular by present- 
ing the following peculiarities : (a) lapilli of a minutely-cellular pumice with 
spherical cells; (&) lapilli with well -developed flow - structure ; (c) lapilli 
consistino' of a pale green serpentinous substance resembling altered 
palagoiiite and probably originally glass; {d) lapilli derived from the de- 
struction of older tuffs ; and (e) lapilli consisting of ejected crystals, especially 
of felspars, sometimes entire, often broken. 
4. They frequently show that they have undergone metamorphism, by 
the development of a pale greenish micaceous mineral between the lapilli, 
the change advancing until the fine tuffs occasionally pass into fine silky 
O O 
schists. 
In my study of the St. David’s district, I was unable to observe 
any evidence that the basic and siliceous tuffs characterize two distinct 
periods of volcanicity. From the foregoing analyses it appears that some 
of the oldest visible tuffs which are seen between Pen-maen-melyn and 
ben-y-foel contain only 48' 11 per cent of silica; while a specimen from 
borth-lisky yielded 72-63 per cent of that ingredient. Specimens taken 
even from adjacent beds show great differences in the percentage of silica, 
as may be seen in the analyses Nos. III. and \ . 
This alternation of basic and siliceous fragmental materials has its 
parallel in the neighl)ouring eruptive rocks, some of which are olivine- 
diabases containing only 45 per cent of silica, while others are highly 
siliceous quartz-porphyries. Put all the siliceous eruptive rocks, so far as I 
have been able to discover, are intrusive, and belong, I believe, to a later 
period than that of the volcanic group; in no single instance do they 
appear to me to be true superficial lava-flows. On the other hand, the 
basic eruptive rocks occur both as contemporaneous sheets and as intrusive 
masses. The presence of both siliceous and basic lavas in the Oanibrian 
Volcanic reservoirs, however, is proA^ed by the character of the tuffs. It 
