i74 
THE SILURIAN VOLCANOES 
BOOK IV 
into the newer, nnmarked by any sucli catastrophe as was once supposed 
to have intervened at the end of each great geological period. There 
are traces, indeed, of slight local disturbances, but these only make the 
general gradual transition more marked. 
Of the vegetation which covered the Silurian lauds hardly anything is 
known. Traces of lycopods and ferns have been detected, and these pi’obably 
formed the chief constituents in what must liave been rather a sombre and 
monotonous Kora. The character of the terrestrial fauna is still hidden 
from us, though we do know that insects winged their way through those 
green flow'erless forests, and that scorpions hkewise harlroured there. 
That these primeval arachnoids were air-breathers is shown by their 
breathing stigmata ; and from the fact that they possessed a well-developed 
poison-gland and sting, we may believe that there were already living at 
the same time other land-animals, possibly of higher grade, on which they 
preyed. But of these ancestral types no actual relics have yet been 
discovered. 
It is the life of the sea-floor that has mainly been chronicled among the 
sedimentary formations. Taking the Silurian system as a whole, we And it 
to be the repository of a remarkably varied assemblage of organisms. 
Among the simpler forms, Eadiolaria deserve especial notice, from their wide 
range in space and time, and the comparative indestructibility of the 
highly-siliceous, fine-grained, flinty strata, which have preserved them in 
abundance and have a wide distriljution over the British Isles. The 
Graptolites, so specially characteristic of the system, range entirely through 
it, and by their successive differences of specific and generic forms, 
furnish a basis for the division of the whole series of rocks into more 
or less definite stratigraphical zones. Hardly less important for purposes 
of correlation are the Trilobites which in the Silurian period reached the 
culmination of their development in regard to number of species and genera. 
These interesting extinct types of crustacean life must have swarmed over 
some parts of the sea-bottom, for their remains abound in its hardened silts. 
The Brachiopods are likewise numerously represented among Silurian strata ; 
and since the vertical range of the species is generally not great, they serve 
as useful guides in fixing stratigraphical horizons. Lamellibranchs, 
Gasteropods, and Cephaloxjods become increasingly numerous and varied as 
we follow the succession of strata from the base to the summit of the 
Silurian system. That there were fishes also in the Silurian seas is 
proved by the occurrence of their remains, more particularly in the higher 
formations. 
From the organic remains which have been preserved in the rocks, it 
may be inferred that the animal life of the globe became more varied in 
Silurian time ; higher types made their appeai’ance, until vertebrates took 
the place of pre-eminence which they have ever since maintained. 
Tlie volcanic activity that had marked the passage of Cambrian time 
in Britain was prolonged into the Silurian period. In North Wales, 
indeed, it is clear that though the eruptions began in the earlier era of 
