internal Structure of the Magnesian Limestone. 77 



with crystals of carbonate of lime, occasionally associated with small but beau- 

 tiful crystals of sulphuret of lead and sulphuret of zinc. As these metallic 

 sulphurets are found with other crystalline materials in cells which have no 

 communication with the surface of the several strata, they must be nearly 

 contemporaneous with the rocks in which they are imbedded. 



The excavations formed for the new Stockton rail-road led to a discovery 

 of great geological interest. In the marl-slate, about two feet above the white 

 sandstone, were found a great many impressions of vegetables and of fish. 

 Of the former, a great many specimens were unfortunately destroyed by the 

 workmen. The only examples which I have seen are now before the Society, 

 and appear to be ferns. Of the fossil fish many good specimens were pre- 

 served, and I have seen portions of at least seven species. Among these the 

 genus Palseothrissum of De Blainville is the most abundant; and the two 

 species Palasothrissum magnum and Pala^othrissum macrocephalum have 

 been identified by that distinguished naturalist. These two species are ex- 

 tremely abundant in the marl-slate of the Thuringerwald ; and it perhaps 

 deserves remark, that in their distortion and mode of preservation they exactly 

 resemble many of the Durham specimens*. 



The superior and more compact beds above described also contain fossils ; 

 among which are two species of Producta, a Spirifer, and a Terebratula. The 

 Producta antiqua of the mountain limestone occurs, though I believe very 

 rarely, in the two quarries above mentioned. 



The zechstein which overlies the marl-slate of Thuringia has been de- 

 scribed as containing Gryphites and Terebratulites, and indeed has been 

 named by M. Voigt a gryphite limestone. Had this account been correct, it 

 would certainly have thrown some difficulty in the way of our classification. 

 Fortunately, however, the Gryphaea aculeata of Schlottheim belongs to the 

 genus Producta of English mineral conchology ; and the fossils of the zech- 

 stein appear to be nearly identical with the corresponding deposit of the 

 British series. 



In order to complete the analogy between the deposits which I am descri- 

 bing and the corresponding formations of Thuringia, it may be remarked, that 

 in the county of Durham the marls which separate the bands of marl-slate and 

 the beds of compact limestone are sometimes bituminous. Traces of bitumen 

 have often been found in the compact limestone ; and at Somerhouse quarry 

 near Denton, a thin-bedded limestone of the same age with the Midderidge 

 series alternates with thin bands of a black micaceous shale, which is suffi- 

 ciently bituminous to be regarded as an impure coal. These, and one or two 



* See Plates VIII. IX. X. XI. & XII. 



