
714 STRATIGRAPHICAL GEOLOGY.  [Boox VI. 
tion shows that the vents ranged themselves in lines or linear groups 
parallel with the general trend of the great central valley. The fact 
that the igneous rocks are succeeded by thousands of feet of sandstones, 
shales, and conglomerates, without any intercalation of lava or tuff, proves 
that the volcanic episode in the history of the lake came to a close long 
before the lake itself disappeared. As a rule the deposits of this 
basin are singularly unfossiliferous, though some portions of them, 
particularly in the Forfarshire (Arbroath) flagstone group, have proved 
rich in fish remains. In Lanarkshire about 5000 feet above the base 
of the system a thin band of shale occurs, containing a graptolite, with 
Spirorbis Lewisii and Orthoceras dimidiatum,—undoubtedly Upper Silurian 
forms. This interesting fact serves to indicate that, though geographical 
changes had elevated the Upper Silurian sea-floor partly into land and 
partly into isolated inland water-basins, the sea outside still contained 
an Upper Silurian fauna, which was ready on any favourable opportunity 
to re-enter the tracts from which it had been excluded (see p. 628). 
The interval of its reappearance seems to have been very brief, however, 
for the band of shale containing these Upper Silurian marine organisms 
is only a few inches thick, and the fossils have not been detected on any 
other horizon. With these exceptions, the fauna of the formation consists 
entirely of fishes and crustaceans. Nine or more species of crustaceans 
have been obtained, chiefly eurypterids, but including one or two 
phyllopods. The large pterygotus (P. Anglicus) is° especially charac- 
teristic, and must have attained a great size, for some of the individuals ~ 
indicate a length of 6 feet with a breadth of 13 feet. There occur also a 
sihaller species (P. minor), two Hurypteri, three species of Stylonurus, and 
abundant clusters of crustacean egg-packets (Parka decipiens). Seventeen 
species of fishes have been obtained, chiefly from the Arbroath flags. They 
belong to the sub-orders Acanthodide and Ostracostei (Fig. 332). One of the | 
most abundant forms is the little Acanthodes Mitchelli. Another common 
fish is Diplacanthus gracilis. 'There occur also Climatius scutiger, C. reticu- 
latus, and C. uwncinatus, Pareaus incurvus, Huthacanthus (four species), 
Cephalaspis Lyellii, and Pieraspis Mitchelli. Some of the sandstones and 
_ shales are crowded with indistinctly preserved vegetation, occasionally 
in sufficient quantity to form thin lamine of coal. In Forfarshire the 
surfaces of the shaly flagstones are now and then covered with linear 
grass-like plants like the sedgy vegetation of a lake or marsh. In Perth- 
shire certain layers occur chiefly made up of compressed stems of 
Psilophyton (Fig. 331). ‘The adjoining land was doubtless clothed with a 
flora in large measure lycopodiaceous. 
The Old Red Sandstone of the northern basin (Lake Orcadie) is typi- 
cally developed in Caithness, where it consists chiefly of the well-known 
dark-grey bituminous and calcareous flagstones of commerce. It rests un- 
conformably upon metamorphosed Lower Silurian schists, and must have 
been deposited on the very uneven bottom of a sinking basin, seeing that 
occasionally even some of the higher platforms are found resting against 
the schists and granites. ‘The lower zones consist of red sandstones and 
conglomerates, which graduate upward into the flagstones. Other red 
sandstones, however, supervene in the higher parts of the system. The 
total depth of the series in Caithness has been estimated at upwards of 
16,000 feet. Murchison was the first to attempt the correlation of the 
Caithness flagstones with the Old Red Sandstone of the rest of Britain, 
