CHAP. X 
ST. DAVID'S 
147 
clear evidence of alternation with the ordinary non- volcanic sediment of the 
time to which they helong. In these respects they are particularly noteworthy, 
for they prove that in the earliest Palmozoic ages the essential features of 
volcanic action were already as well developed as in any subsequent epoch 
of geological history. 
The volcanic group of St. David’s attains a visible thickness of about 
1800 feet. Its upper part graduates upward into purple and green Lower 
Cambrian sandstones. The base of the group is not seen owing to the 
plicated structure of the district. Hence the total thickness of volcanic 
material cannot be determined, neither can we tell on what it rests, whether 
on a still lower sedimentary series or on some platform of pre-Cambrian 
rocks. 
The structure of the group, notwithstanding all that has been written 
about it, has never yet been adequately worked out. The unfortunate and 
barren controversy about supposed pre-Cambrian rocks at St. David’s has 
tended to obscure the real importance of these rocks as the oldest well- 
preserved record of volcanic action in Britain. They deserve to be carefully 
surveyed on maps of a large scale, in the same detailed manner as has been 
so successfully applied to the elucidation of younger volcanic tracts. Until 
such detailed investigation is made, any account of them which is given can 
be little more than a general outline of the subject. The following description 
is the result of my examinatioTi of the ground in company with my colleague 
i^Ir. B. N. Beach, and afterwards with the late Mr. W. Topley.' A few 
additional observations, from the subsequent exploration of Professor Lloyd 
"Morgan," are incorporated in the narrative. 
The geologist who traces these St. David’s rocks in the field cannot 
fail to be struck with their general resemblance to volcanic masses of later 
Lalieozoic date. Many of the lavas and tuffs are in ontward characters 
firrite indistinguishable from those of the Lower Old Eed Sandstone and 
Carboniferous systems of Britain. So many points of detail may be observed 
to be common to the Paheozoic eruptive rocks all over the country from 
the Cambrian to the Permian periods as to indicate that volcanic 
phenomena must have recurred under much the same conditions throughout 
Palseozoie time. 
By far the larger part of the Cambrian volcanic group of St. David’s 
consists of bedded tuffs, though a few lavas are interstratified in it, 
particularly towards the top. The whole has subsequently been invaded by 
acid protrusions, and lastly by basic dykes. 
1. Bedded Tuffs and Lavas . — The tuffs, which are the predominant 
nrembers of the volcanic group, present many varieties of colour, from dark 
purple, through tints of brick-red and lilac, to pale pink, yellow and creamy 
W’hite, but not unfrequently assume various shades of dull green. They 
* Quart. Joiirn. Oeol. Soe. vol. xxxix. (1883), p. 294 et seq. While the essential parts of the 
investigation are given in the following pages, I would refer the reader to this paper for details 
not^tranaferred to tlie present volume. 
^ Op. ait. vol. xlvi. (1890), p. 241. 
