III. CARBONIFEROUS SYSTEM.
The Carboniferous system occupies fully three-fourths of the district, and has a thickness of about 7000 feet. It is divisible into four distinct formations, all of which are conformable to each other, and marked by the occurrence of Stigmaria ficoides.
1. Tuedian group or formation I applied, in 1856, to a series of beds, intermediate between the Mountain Limestone and the Upper Old Red Sandstone, having an aggregate thickness of about 1000 feet, and consisting of grey, greenish, and lilac shales, thin beds of argillaceous and cherty limestones, a few buff magnesian limestones, and of sandstones and slaty sandstones, several of which are red, and some, near the bottom of the series, of considerable thickness. Several, too, of the shales and sandstones are calcareous; so that, though the limestones are thin and impure, there is a considerable quantity of calcareous matter diffused throughout the formation. The limestones are too impure to burn into lime, excepting a curious bed of magnesian limestone, near Carham, which is composed of carbonate of magnesia 44, carbonate of lime 49.6, silica 4, and peroxide of iron 1.2, alumina 1; and in this are nodules of red and grey chert, analogous to flints in chalk. Stigmaria ficoides, Lepidodendra, coniferous trees, reed-like stems on which are Spirorbis, a Sphenopteris, and other carboniferous plants, occur both in the sandstones and shales; but there are no beds of coal. The Fauna generally consists of Rhizodus Hibberti, Gyracanthus, and other fish; and of mollusks allied to Modiola. Fresh-water or lacustrine conditions are generally indicated; for few distinct marine organisms appear, and where discovered were accompanied by plants in a fragmentary condition, which seem to have been swept into a shallow estuary. One bed of shale, with irregular layers of impure limestone, contains broken trunks of Pinus primaeva and antiqua, showing internal structure, and associated with Orthoceras multiseptum (n.s.), Murchisonia Verneuliana, Pleurotomaria, and a few other marine forms. Brachiopods and Encrinites, which are so abundant in the Mountain Limestone, are almost
