7 
740 STRATIGRAPHICAL GEOLOGY. — [Boox VI. 
argillaceous limestone or cement-stone. Seams of gypsum occasionally 
appear. These strata are, on the whole, singularly barren of organic 
remains. ‘They seem to have been laid down with great slowness, and 
without disturbance, in enclosed basins, which were not well fitted for 
the support of animal life, though fragmentary plants serve to show 
that the adjoining slopes were covered with vegetation. In the basin of 
the Firth of Forth, however, the group presents a different lithological 
aspect and is abundantly fossiliferous. It there usually consists of 
yellow, grey, and white sandstones, with blue and black shales, clay- 
ironstones, limestones, “‘ cement-stones,” and occasional seams of coal. 
The sandstones form excellent building stones, the city of Edinburgh 
having been built of them. Some of the shales are so bituminous as to 
yield, on distillation, from 30 to 40 gallons of crude petroleum to the ton — 
of shale; they are consequently largely worked for the manufacture of 
mineral oils. The limestones are usually dull, yellow, and close-grained, 
in seams seldom more than a few inches thick, and graduate by 
addition of carbonate of iron into cement-stone; but occasionally they 
swell out into thick lenticular masses like the well-known limestone 
of Burdie House, so long noted for its remarkable fossil fishes. This 
limestone appears to be mainly made of the crowded cases of a small 
ostracod crustacean (Leperditia Okeni, var. Scoto-Burdigalensis). The coal- 
seams are few and commonly too thin to be workable, though one of them, 
known as the Houston coal, has been mined to some extent in Linlith- 
gowshire. The fossils of the cement-stone group indicate an alternation 
of fresh or brackish water and marine conditions. They include nume- 
rous plants, of which the most abundant are Sphenopteris afinis (Fig. 
345), Lepidodendron (two or three species), Lepidostrobus variabilis (Fig. 
348, b), Araucarioxylon. Some of the shales near Edinburgh have afforded 
a few specimens of a true monocotyledon allied to the modern Pothos 
(Pothocites Grantoni). Ostracod crustaceans, chiefly the Leperditia above © 
mentioned, crowd many of the shales. With these are usually associated 
abundant traces of the presence of fish, either in the form of coprolites, 
or of scales, bones, plates, and teeth. The following are characteristic 
species : Llonichthys striolatus, EZ. Robisoni, Rhadinichthys ornatissimus, Nema- 
toptychius Greenockii, Eurynotus crenatus (Fig. 344), Rhizodus Hibberti, 
Megalichthys sp., Gyracanthus tuberculatus, Ctenoptychius pectinatus. At 
intervals throughout the group marine horizons occur, usually as shale 
bands marked by the presence of such distinctively Carboniferous Lime- 
stone species as Spirorbis carbonarius, Discina nitida, Lingula squamiformis, 
Bellerophon decussatus, and Orthoceras cylindraceum. 
The Cement-stone group of the basin of the Firth of Forth con- 
tains a great number and variety of associated volcanic masses. At the 
time when it was accumulating, the region of shallow lagoons, islets, and 
coal-growths was dotted over with innumerable active volcanic vents. 
The eruptions continued into the time of the Carboniferous Limestone, 
but ceased before the deposition of the Millstone Grit. The lavas are 
chiefly varieties of basalt-rocks, sometimes coarsely crystalline and even 
granitoid in texture, and graduating through intermediate stages to true 
close-grained compact basalts, which neither externally nor in micro- 
scopic structure differ from basalt of Tertiary date. Among them also 
are felsites and porphyrites. The tuffs present many varieties, one of the 
most interesting being an ancient form of palagonite-tuff. 
The Carboniferous Limestone series of Scottish geologists, probably 
1 See Trans. Roy. Soc. Hdin, xxix. p. 437, and ante, p. 547, et seq. 
