Paleo-Geological and Geographical Maps of the British Islands, 263 
beds may be inferred to be of oceanic origin. Cambrian rocks of this type are found 
in the east of Ireland, with Oldhamia, a sertularian zoophyte, and annelid tracks 
and borings, such as those of Histvoderma. In North Wales and Shropshire they 
have yielded trilobites,* while the Upper Cambrian beds are rich in marine forms, 
At St. David’s, Dr. Hicks has brought to light several genera of trilobites in beds 
contemporaneous with those of the Longmynd and Harlech group of North Wales. 
These rocks also are found in Charnwood Forest, in Leicestershire,t where they are 
in some places altered or metamorphosed, and associated with trap rocks; in the 
Ardennes mountains, on the borders of France and Belgium, where they consist of 
quartzites, quartzschists, and schists, with Oldhamia radiata (one of the Irish 
species), Dictyonema sociale, Linyula, and tubes or impressions of annelids.{ In the 
Systéme Salmien, forming the upper division of the series, trilobites of the genus 
Paradoxides have been discovered by M. Malaise.§ 
These rocks also occur in Normandy and Brittany, consisting of green slates 
and grits, resting on gneiss and schist, which is probably of Laurentian age. 
Regarded as a whole, the Cambrian beds of the region now described are 
clearly of marine origin, and present—both in lithological characters, and from the 
occurrence of a marine fauna—a marked dissimilarity to the beds of the Caledonian 
type. 
Puate XXIV. 
The Lower Silurian Period. 
With the commencement of the Lower Silurian period,|| the ocean resumed the 
dominion it had partially lost during the preceding Cambrian period ; and as time 
went on, the entire area of the British Islands and adjoining parts of Europe 
became submerged and covered with sediment. It may be confidently affirmed, 
that there is not a square mile over this region which was not originally buried 
beneath strata belonging to the Lower Silurian period. The old archzean ridge was 
covered by strata still in existence; and even the Cambrian and Laurentian rocks 
of the north Highlands of Scotland were, in the opinion of Sir A. C. Ramsay, 
submerged and buried under the accumulating piles of these strata before that era 
passed away.1 Land was, however, probably not far away, and its position was to 
the north-west of the British Isles. 
* Discovered by Mr. Salter in the Longmynd beds. 
+I regret I cannot agree with Dr. Hicks and several other distinguished geologists in regarding these 
beds otherwise than of Cambrian age, to which they were originally referred by Professor Jukes. 
{ Dr. Mourlon, “ Géologie de la Belgique,” t. 1., p. 31. 
§ Dalimier, Bull. Soc. Géol. France, 2 Ser., vol. xx. 
|| 1 assume the base of the Lower Silurian series to be the Tremadoc slate or (in their absence) the 
Arenig beds, forming the lower part of the Llandeilo group. 
q «« Phys. Geog. and Geol. of Great Britain,” 5 edit., p. 87. 
