476 
ELEMENTS OF GEOLOGY. 
1843 , strata now ascertained to be of the same age as largely 
developed in the Arenig mountain, in Merionethshire; and 
the Skiddaw slates in the Lake-District of Cumberland, stud¬ 
ied by the same author, were of corresponding date, though 
the number of fossils was, in both cases, too few for the de¬ 
termination of their true chronological relations. The sub¬ 
sequent researches of Messrs. Sedgwick and Harkness, in 
Cumberland, and of Sir R. I. Murchison and the Government 
surveyors in Shropshire, have increased the species to more 
than sixty. These were examined by Mr. Salter, and shown 
in the third edition of ^^Siluria” (p. 52 , 1859 ) to be quite 
distinct from the fossils of the overlying Llandeilo flags. 
Among these the Obolella plumbea^ ^glina binodosa, Ogygia 
Sely:)y7iii^ and Duly- 
m ograpsics gem mus 
Fig. 564. 
Didymogra2)Sus geminus, Hisinger, sp. Sweden. istic. 
But, although the species are distinct, the genera are the 
same as those which characterize the Silurian rocks above, 
and none of the characteristic primordial or Cambrian forms, 
presently to be mentioned, are intermixed. The same may 
be said of a set of beds underlying the Arenig rocks at Ram¬ 
say Island and other places in the neighborhood of St. Da¬ 
vid’s. These beds, which have only lately become known to 
us through the labors of Dr. Hicks,* present already twenty 
new species, the greater part of them allied generically to 
the Arenig rocks. This Arenig group may therefore be 
conveniently regarded as the base of the great Silurian sys¬ 
tem, a system which, by the thickness of its strata and the 
changes in animal life of which it contains the record, is 
more than equal in value to the Devonian, or Carboniferous, 
or other principal divisions, whether of primary or secondary 
date. 
It would be unsafe to rely on the mere thickness of the 
strata, considered apart from the great fluctuations in or¬ 
ganic life which took place between the era of the Llandeilo 
and that of the Ludlow formation, especially as the enormous 
pile of Silurian rocks observed in Great Britain (in Wales 
more particularly) is derived in great part from igneous ac¬ 
tion, and is not confined to the ordinary deposition of sedi¬ 
ment from rivers or the wafete of cliffs. 
In volcanic archipelagos, such as the Canaries, we see the 
most active of all known causes, aqueous and igneous, simul¬ 
taneously at work to produce great results in a compara- 
* Trans. Brit. Assoc., 1866. Proc. Liverpool Geol. Soc., 1869. 
