\ 
Ta STRATIGRAPHICAL GEOLOGY. [Book VI. 
dark anthracitic shale which have heen traced across nearly the whole _ 
breadth of the country. ‘These shales, crowded with graptolites of recog- 
nizable Llandeilo forms (Climacograptus teretiusculus, Diplograptus pristis, 
and Graptolithus sagittarius being particularly abundant), were deposited 
over wide areas of sea-bottom. It is remarkable that wherever they 
appear the graptolites come with them, as if these organisms could only 
flourish on the black carbonaceous mud. The persistence of the graptolitic 
fauna is shown by the fact that many of the same species occur in the upper 
black shales at a vertical distance of more than 10,000 feet above the 
horizon of the lower shales (p. 629). Crustacea are exceedingly rare, but 
two phyllopods, Discinocaris Browniana and Peltocaris aptychoides, occur ; 
while from Dumfriesshire two obscure trilobites are referred doubtfully to 
Enerinurus and Phacops. The vast thickness of sandy, gritty, and shaly 
unfossiliferous strata is the distinguishing feature of the Lower Silurian 
series in the south of Scotland.1. The Caradoc or Bala group lies un- 
conformably upon the upper parts of the Llandeilo rocks. It contains in 
the eastern districts some calcareous conglomerates which here and there 
swell out into local masses of hmestone. In the south-west of Ayrshire 
the limestones attain considerable dimensions. In these calcareous 
bands numerous Caradoc species have been fuund, among them Cheirurus — 
gelasinosus, Encrinurus punctatus, with species of Ilzenus and Asaphus, Orthis 
calligramma, O. confinis, Lepteena sericea, Maclurea, and such corals as 
Heliolites, Favosites, Omphyma, and Strephodes. In the same district 
certain shales and sandstones full of Caradoc fossils are overlaid with 
sandstones, shales, and conglomerates containing Pentamerus oblongus, 
Atrypa hemispherica, Meristella angustifrons, Lichas laxatus, Petraia elongata, 
Nidulites favus, and numerous other fossils which indicate the horizon of 
the Llandovery rocks. 
The Highlands of Scotland, as above (p. 583) stated, consist mainly of 
crystalline rocks—gneiss, mica-schist, chlorite-schist, clay-slate, quartz- 
rock, schistose flagstone, and many others, which from the discovery of 
recognizable fossils near their base have been shown to be metamorphosed 
Lower Silurian rocks. As this deduction possesses very great impor- 
tance in theoretical geology, particularly in relation to the history of 
metamorphism and metamorphic rocks, it is desirable that the true 
geological horizon of fossils found below so vast a pile of crystalline 
schists should be precisely determined. Fortunately the number and 
good preservation of the specimens allowed the determination to be satis- 
factorily made by Salter, who declared his conviction that they were 
unequivocally Lower Silurian, and bore a most remarkable resemblance 
to a group of fossils from the Lower Silurian rocks of North America. 
Five of the species he regarded asidentical with known American forms 
( Orthoceras arcuoliratum, Hall; Orthis striatula, Emmons ; Ophileta compacta, 
Salt. ? Murchisonia gracilis, Hall; M. bellicincta, Hall), 4as representative, 
3 doubtful, and 1 new genus, found also in Canada. ‘“ That this truly 
North American assemblage,” he remarks, ‘‘should be found in the ex- 
1 Mr. Charles Lapworth, who has deyoted much time to the study of the graptolites 
of these rocks, has come to the conclusion that what is here termed the Moffat Shale 
group, and regarded as merely a subordinate member of a thick series of sandy and 
generally unfossiliferous strata, represents the whole series of strata from the Llandeilo 
up into the Upper Silurian formations; that is to say, somewhere about a half of the 
whole of the Silurian system is contained in a group of shales and sandstones, sometimes 
less than 200 feet thick ! 
