Part IL. Sect. ii. § 2.) | SILURIAN. 671 
typical subdivisions in Wales. The Skiddaw slates are black or dark- 
grey argillaceous, and in some beds sandy, rocks, often much cleaved, 
though seldom yielding workable slates, sometimes soft and black like 
Carboniferous shale. As a rule they are singularly unfossiliferous, but 
in some of their less cleaved and altered portions they have yielded 
about 40 species of graptolites (chiefly of the genera Didymograptus, 
Diplograptus, Dichograptus, Tetragraptus, Phyllograptus, and Climacograptus), 
Lingula brevis, traces of annelides, a few trilobites (Aiglina, Agnostus, 
Asaphus, &c. ), some phyllopods (Car yocaris), and remains of plants 
(Buthotrephis, &c.). In many places the slates have been metamorphosed, 
passing into chiastolite-slate, mica-schist, andalusite-schist, &c., with pro- 
trusions of granite, syenite, and other crystalline rocks (p. 579). Towards 
the close of the long period represented by the Skiddaw slates, volcanic 
action manifested itself, first by intermittent showers of ashes and 
streams of lava, which were interstratified with the ordinary marine 
sediment, and then by a more powerful and continuous series of 
explosions, whereby a huge volcanic mountain or group of cones was 
piled up above the sea-level. The length of time occupied by this 
volcanic episode in Cumbrian geology may be inferred from the fact that 
all the Llandeilo and nearly all the Bala beds are absent here. The 
volcanic island slowly sank into a sea where Bala organisms flourished. 
Among these we find such familiar Bala species as Favosites fibrosa, 
Helioliies interstinctus, Cybele verrucosa, Leptzena sericea, Orthis Actonix, O. 
biforata, O. caligramma, O. elegantula, O. porcata, and Strophomena rhom- 
boidalis. These organisms and their associates gathered on the sub- 
merged flanks of the sinking volcano into a bed of limestone—the 
Coniston limestone—which can still be traced for many miles through 
the Westmoreland hills, as the Bala limestone which it represents can 
be followed through the volcanic tracts of North Wales. The Coniston 
limestone is covered by certain flags and grits which from their organic 
remains are referred to the Upper Silurian series. 
In the south of Scotland, according to the detailed researches of the 
Geological Survey, the Lower Silurian formations are represented by the 
subjoined groups of strata in descending order : 
Sandstones and conglomerates, Girvan valley . . =Liandovery. 
Conglomerates, grits, shales, and lenticular bands of 
limestone, Peeblesshire, Dumfriesshire, S.W. Ayn Cond or Bala. 
shire, sometimes 2000 ft. 
Carsphairn group, coarse pebbly orits and ereywacke,) 
1200 ft. 
Upper Black Shale, with ‘eraptolites, 550 ft. 
Lowther group, olive, grey, and blue shales, and 
sandstones, +000 ft. ‘ 
een group, greywacke and shale, with band of 
e conglomerate, 3500 ft. . . 
Queensberry group, massive ereywackes and orits, with = Llandeilo (14,000 ft.). 
occasional conglomerate bands and some shales, 

4500 ft. . 
Lower or Moffat Black Graptolite Shale group, 
200-400 ft. 
Ardwell croup, brown flags, ereywackes, ‘and shales, 3 
sometimes purplish and red; base not seen if 
As a whole these strata are singularly barren of organic remains. 
Most of the fossils which the Llandeilo groups contain lie in the bands of 
