378 Murchison’s Sitluria. 
the world as too strong to be easily set aside. The thickness of 
the Lower Silurian rocks may be increased to an almost indefinite 
extent by intercalated trappean masses and volcanic grits a 
conglomerates. This is not only exemplified in North Wales, 
where the trappean and Bala groups, which are made of a suc- 
cession of black slates, with occasional bands of sandstones and 
grits and great masses of trappean rocks and a few bands of lime- 
stone containing Lower Silurian fossils, reach the enormous devel- 
opment of 24,000 feet, but in this country, in the basin of Lake 
Superior, where the lower member of the Silurian, the equiv- 
alent of the Potsdam sandstone, which in its normal condition, 
at a distance from volcanic disturbances, is composed of a few 
hundred feet of fine-grained sandstone, in the region of trappean 
outbursts, acquires a thickness, with its associated trappean rocks 
and conglomerates, of at least three miles. | ; 
: e whole course of geological investigations since the publi- 
cation of the “ Silurian System” seems to us to have accumulated 
so much evidence in favor of our having reached in the Lower 
Silurian the commencement of the existence of organic life on 
the globe, that this is to be accepted as one of the great results of 
the science, at least until some vestiges of a Fauna or Flora of 
an older type shall have been discovered. ‘There can be no fact 
more unequivocally made out than the extraordinary resemb 
everywhere manifested in the Fauna of the oldest fossiliferous 
rocks. Throughout the whole extent of the earth’s surface which 
has been explored, we find that there is in the Lower Paleozoic 
types, as we descend in the series, a more near approach to iden- 
tity in fossils of remote localities but of the same geological period. 
In the Bohemian basin, whose geology has been so elaborately 
worked out by Barrande, this geologist describes, at the base 
the fossiliferous beds, a series of schists, conglomerates and un- 
doubted sedimentary strata, in which not a trace of orgame life 
ean be discovered, and which he does not hesitate to designate a5 
analogous bodies. Of the seven genera of trilobites above —- 
‘tioned, only one, Agnostus, is found in an overlying group; ®” 
ruption of a vast mass 
gn ? 
the others disappear forever 
ry which separates the “ 
in 22 ao 
