156 The Auiericau Geolo(jist. March, 1895 
ti confused manner involved with masses of eruptive roeks, 
granites, felsytes and dolerytes, embracing marbles, schists, 
gneisses and basic eruptives. 
It is apparent, therefore, that in Europe any stratigraphic 
plane which may be assumed for the base of the Cambrian 
would be wholly artificial ; what would be the base at one 
place, owing to a progressive subsidence which, according to 
Dr. Hicks, has been found to have been in progress during the 
whole of Cambrian time, would not be the base in another. 
The Middle Cambrian (Menevian) lies on the Archean in the 
Anglesea area, and in several other places in Wales, the very 
base being conglomerates with a marked discordance on the 
crystalline schists. In Shropshire the lowest recognizable 
Cambrian strata carry Olenellus, and they lie upon a series of 
volcanic materials, made up both of lavas and fragmental 
ejections which have been classed as pre-Cambrian, though not 
on the best of evidence. Such volcanic products are found to 
underlie the Upper Cambrian in Warwickshire.* Dr. Hicks 
has supposed, from a survey of all these facts, that a subsid- 
ence of the continental areas, both eastern and western, 
brought the Cambrian sea further and further upon the land; 
that this was accompanied by such volcanic and other physi- 
cal changes that not only was the fauna extinguished in the 
oceanic waters thus affected, but that volcanic ejections were 
from time to time interbedded in the Cambrian strata. Thus 
a kind of general similarity of lithology and succession of 
parts was imprinted on these strata on both sides of the At- 
lantic. It has been remarked by Sir Archibald Geikie that 
"the rocks of the Cambrian sj^stem present considerable uni- 
formity of lithological character over the globe." Probably 
the progenitors of the Cambrian fauna lie buried under all the 
later strata in tlie basin of the Atlantic. The oscillating con- 
tinental borders, the loci of the nu)st frequent tiexures of the 
crust and of the escape of molten rock from volcanic vents, 
would thus be re-peopled in periods of quiet by immigration 
of new species from the adjacent ocean, f thus making a rec- 
*Lapwouth, Geological Mauazinc. 18S(). p. :]21. 
fHiCKS, Quarl. .Tour. (leol. Soc, xxxT. p. .l.l'i. Natural Scii'iicc, \ol. 
v. DfC. 18!)4. 
