TRANSACTIONS OF SECTION C. 803. 
2. On Pre-Cambrian Rocks occurring as Fragments in the Cambrian Con- 
glomerates in Britain.! By Henry Hicks, M.D., F.R.S., F.G.S. 
In this paper the author indicates by a table the contents of the basal Cambrian 
conglomerates in several areas in Britain, where he and others have claimed that 
Pre-Cambrian rocks are now exposed. He shows, on the authority of such eminent 
petrologists as Professor Bonney and Mr. T. Davies, that rock-fragments which 
have been collected from the conglomerates in various districts by Professor Hughes, 
Dr. Calloway, and himself, have been proved to be identical in character in the 
minutest microscopical details with some peculiar granitoid rocks, and some basic 
and acid volcanic rocks, schistose rocks and porcellanites, which have been described 
by them as Pre-Cambrian rocks in those areas. He further shows that in some 
places the conglomerate is almost entirely made up of rolled fragments from imme- 
diately underlying rocks. At Ramsey Island, and Treffgarn in Pembrokeshire, at 
Bangor, and near Llanberis and Bethesda in Carnarvonshire, where the Cambrian 
conglomerates rest on Felsites and old Rhyolites, more than three-fourths of the 
pebbles, which are frequently of very large size, have been derived from the imme- 
diately underlying rocks. Near St. David’s, and at other places where the con- 
glomerates rest on various altered volcanic tufts, a large number of the pebbles have 
been derived from those tuffs after they had been cleaved and otherwise changed 
into their present condition. At Porthclais, Chanter’s Seat, and Porth Melyn, near 
St. Dayid’s, a large number of the pebbles (mostly of small size) and the mixture of 
broken quartz and felspar, of which some of the beds are almost entirely composed, 
could only have been derived from the underlying granitoid rocks (Dimetian). The 
author shows that near Llanfaelog and Llanerchymedd, in Anglesea, very large: 
pebbles of the underlying granitoid rocks are abundant in the overlying Cambrian 
conglomerates, and that at Twt Hill, near Carnarvon, the matrix and many of the 
pebbles must undoubtedly have been derived from the underlying granitoid rocks. 
Table showing the rocks which have been found in the Cambrian 
Conglomerates in different areas. 
4 g S : 2 2 ey 
TE Ee an RS Si Ong AP ae 7a fees 
Rocks 2h eal Ae ul obaivn Geo) Bach oe 
ag hag ua 2 is hoa tat = a BA 
Ay = S as n 64 a 
Granitoid (Granite, Pegmatite, i. 
i 05) Io a : - : x x x x Wx x x 
Quartz porphyry x — x x - 
Felsite ; ; . : x x x x x 
Rhyolite, Dacite, and Andesite x x x x x 
Diorite and Syenite . : .| — — — = — x x 
Diabase and Basalt . x x x x x x x 
Gneiss : : 8 - — — — x x x x 
Sericite schist . : x x x x x x 
Chlorite schist . ; x x 7 x x x 
Hornblende schist —_ — = x — x x 
Mica schist x x x x x x x 
Quartz schist 5 3 ‘ x x x x x x x 
Volcanic fragmental (Acid and 
Basic) . 5 ; x x x x x x 
Porcellanite - x x x x x 
Clay slate . x x x x x x x 
Quartzite . x x x x x x x 
Sandstones x x x x x x x 
Calcareous — —- — x — — x 
Ferruginous . = - x x x x x x x 
Quartz, Jasper,&ke. . x x x x x x x 
' Published in extenso in Geol. Magazine for November 1890. 
