Ch. XXIII.] 



PEEMIAN LIMESTONES. 



351 



I shall proceed, therefore, to treat briefly of these subdivisions, begin- 

 ning with the highest, and referring the reader, for a fuller description of 

 the lithological character of the whole group, as it occurs in the north 

 of England, to a valuable memoir by Professor Sedgwick, published in 

 1835.^' 



Crystalline or concretionary limestone (No. 1). — This formation is seen 

 upon the coast of Durham and Yorkshire, between the Wear and the 

 Tees. Among its characteristic fossils are Sckizodus Schlotheimi (fig, 

 444) and Mytilus septifer (fig. 446). 



Fig. 444. 



Fiff. 445. 



Flo:. 446. 



ScMzodns Schlotheimi, Geinitz. 

 Crystalline limestone, Permian. 



The hinge of Sehizodus 



truncatus, King. 



Permian. 



IFytilus septifer, King. 

 Syn. Modiola acuminata. 



James Sow. 

 Permian crystallioe lime- 

 stone. 



These shells occur at Hartlepool and Sundei-land, where the rock as- 

 sumes an oolitic and botroidal character. Some of the beds in this 

 division are ripple-marked ; and Mr, King imagines that the absence of 

 corals and the character of the shells indicate shallow water. In some 

 parts of the coast of Durham, where the rock is not crystalline, it con- 

 tains as much as forty-four per cent, of carbonate of magnesia, mixed 

 with carbonate of lime. In other places, — for it is extremely variable in 

 structure, — it consists chiefly of carbonate of lime, and has concreted 

 into globular and hemispherical masses, varying from the size of a mar- 

 ble to that of a cannon-ball, and radiating from the centre. Occasionally 

 earthy and pulverulent beds pass into compact limestone or hard granu- 

 lar dolomite. The stratification is very irregular, in some places well- 

 defined, in others obliterated by the concretionary action which has re- 

 arranged the materials of the rocks subsequently to their original 

 deposition. Examples of this are seen at Pontefract and Ripon in 

 Yorkshire. 



The hrecciated limestone (No. 2) contains no fragments of foreign 

 rocks, but seems composed of the breaking-up of the Permian limestone 

 itself, about the time of its consolidation. Some of the angular ma.jses 

 in Tynemouth Cliff are 2 feet in diameter. This breccia is considered 

 by Professor Sedgwick as one of the forms of the preceding limestone, 

 No. 1, rather than as regularly underlying it. The fragments are angu- 

 lar and never water-worn, and appear to have been re-cemented on the 

 spot where they were formed. It is, therefore, suggested that they may 

 have been due to those internal movements of the mass which produced 

 the concretionary structure ; but the subject is very obscure, and after 



* TraDS. Geol. Soc. Lond. Second Series, vol. iii. p. 37. 



