Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne (1867).djvu/35

Rh Pierce Bridge, on the Tees: the base line is obscured by drift, but seems to curve away from Hartlepool westward to Chilton, and thence southward to near Coniscliffe. It occupies an area of about 230 square miles, and has a thickness of about 600 feet.

The Magnesian Limestone strata rise to the westward and south-west, and their outcrop over the Coal Measures is seen marked, in the features of the country, by a pretty bold escarpment running in a broken line of eminences of moderate elevation, but nearly on the same level throughout the range. To the west is the wide and tame region of the Coal Measures, but eastward are low undulating hills, intersected by picturesque and beautiful denes and ravines, in some of which rare plants find sheltered habitats, as in Castle Eden Dene, where grows the much-prized Cypripedium calceolus. Excepting, however, when covered with detritus, the soil immediately over the Magnesian Limestone is far from being rich or productive.

No formation has been more carefully examined than the Magnesian Limestone of our district. The important researches of Sedgwick have explained its physical characters, and its range and relations; and to Professor Philips, Professor King, Mr. Howse, and Mr. Kirkby, we are indebted for an extensive knowledge of its fossils. The result of these labours has been the determination and description of about one hundred and twenty species of marine organisms. The successive subordinate divisions of this formation have been described by Sedgwick, King, and Howse, to whose memoirs reference may be made for details, and for the views of these authors. Beginning at the lowest we have the following succession of groups of strata:—

1. Marl Slate, a calcareous shale but slightly magnesian, seldom more than 3 feet in thickness, but remarkable for the number of fish-remains with which it is loaded. Thirteen species have been found in it—nine of the genus Palcsoniscus, the others being species of Platysomus, Acrolepis, Pygopterus, and Coelacanthus. With these are Discina nitida and Lingula mytiloides;