NORTH UM15EELANI> AKD DtlRHAM. 19 



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 escarp- 

 ment running in a broken line of eminences of moderate eleva- 

 tion, 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. 



!N^o formation has been more carefully examined than the Mag- 

 nesian 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 know- 

 ledge 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 divis- 

 ions 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 Pal<soniscus, the others 

 being species of Platysomus, Acrolepis, Fyyopterus, and Cala- 

 canthtis. "With these are Discina nitida and Lingula mytiloides; 



•Sedgwick; Trans, of Geological Society, Vol. Ill, Part 1, 1829; King's Monograph of 

 Permian Fossils, 1848; Howse on the Permian System, in Annals of Nat. Hist., 1857. 



