Dr. H. Hicks—Pre-Cambrian Rocks of Wales. 397 
carried on by him in conjunction with myself at St. Davids, in 
Pembrokeshire, we had found “that the mass of igneous rock which 
forms the backbone of the St. Davids peninsula, and which supports, 
without penetrating them, the shallow water accumulations of the 
older Cambrian around it,” was a ‘“‘ part of the old Pre-Cambrian 
land,” and that ‘as the purple rocks, sandstones, and slates of the 
whole Lower Cambrian division are thrown up at high angles, all 
but vertical on either flank of this mass, which forms the axis of 
‘the whole country, there is no difficulty in studying its behaviour in 
contact with the Cambrians.” Moreover, it was stated that “if it 
were an intrusive trap of later date it would penetrate them here 
and there, or at least alter them at the point of contact. On the 
contrary, wherever the boundary can be seen, steatitic and felspathic 
schists unaltered, and beds of thick conglomerate mark the line and 
are often very conspicuous. ‘These conglomerates of quartz rock, 
jasper, felstone, etc., may or may not have been derived from the 
immediate neighbourhood. They are traceable along the south and 
north sides of the trap region, and are followed by sandstones of 
various degrees of coarseness, but indicating by the ripple-mark, as 
well as the coarse material, that they were accumulated in shallow 
water ; and as we know that pebbles often as large as swans’ eggs, 
are not carried far out to sea, but mark either a submarine shoal or 
a coast-line, we are compelled to assign them to a source near at 
hand.”! The above remarks (Brit. Assoc. Report and Gurox. Maa. 
1864) were written as already stated twenty-nine years ago, and 
the evidence since obtained has confirmed in a marked degree the 
main conclusions we then arrived at. The great mass referred to 
was then, and still remains, marked on the map of the Geological 
Survey as syenite and felstone, chiefly of Lower Silurian age. The 
view put forward by Mr. Salter and myself that it was of Pre- 
Cambrian age was, as is well-known, most strongly opposed by the 
chiefs of the Geological Survey, and up to a very recent period they 
1 The basal Cambrian conglomerates at St. Davids, which in places are over a 
hundred feet in thickness, often contain very large pebbles. This day (Aug. 17th) I 
measured two well-rounded blocks at the base of the conglomerate where it rests on 
the upturned edges of the Pebidian strata at Whitesand Bay, which gave the follow- 
ing dimensions :— 
Quartzite pebble with a slightly purplish tinge, 18 inches in length, 12 in width, 
and 9 in thickness. Quite near to the above was a well-rolled block of vein quartz, 
17 inches in length, 16 in width, and 11 in thickness. On the same face dozens of 
pebbles 6 to 8 inches across were also visible; and amongst them were numerous 
somewhat smaller pebbles of slaty and compact volcanic ash, porcellanite, schistose 
felstone, etc., which had undoubtedly been derived from the underlying Pebidian 
rocks. The enormous quantity of quartzite, and the frequent presence of fragments 
of quartzose schists, in the conglomerate prove clearly that there must be a hidden 
ridge of such rocks (Arvonian) near at hand. The quartz-grains and bits of felspar 
which formed these quartzites and schists were primarily derived fram Granitoid 
rocks (Dimetian), such as are now seen at St. Davids, and on the coast at Poulthikey. 
In addition, therefore, to showing the marked unconformity on the Pebidian, these 
conglomerates yield indisputable proof of the exposure of three Pre-Cambrian groups 
in the St. Davids area when they were deposited. No amount of ingenious argu- 
ment can alter these facts, and the evidence is perfectly clear to those who desire to 
reach it. 
