Ch. XXHI.] PERMIAN LIMESTONES. 459 



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

 ginning with tlie highest, and referring the reader, for a fuller descrip- 

 tion 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 Schizodus Schlo- 

 theimi (fig. 492) and Mytilus septifer (fig. 494). 



Tig. 492. Fig. 493. Fig. 494. 



Schizodus Sc7ilothei?ni, Geinitz. The hinge of ScMzodus Mytilus septifer, King. 



Crystalline Limestone, Permian. truncatus, King. Syn. Modiola acuminata. 



Permian. James Sott. 



Permian crystalline lime- 

 stone. 



These shells occur at Hartlepool and Sunderland, where the rock 

 assumes an oolitic and botryoidal character. Some of the beds in this 

 division are ripple-marked; aud 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 contains as much as 44 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 con- 

 creted into globular and hemispherical masses, varying from the size 

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

 Occasionally earthy and pulverulent beds pass into compact limestone 

 or hard granular dolomite. The stratification is very irregular, in 

 some places well defined, in others obliterated by the concretionary 

 action which has rearranged the materials of the rocks subsequently 

 to their original deposition. Examples of this are seen at Pontefract 

 and Rip on in Yorkshire. 



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

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

 stone itself, about the time of its consolidation. Some of the angu- 

 lar masses in Tynemouth Cliff are 2 feet in diameter. This breccia 

 is considered by Professor Sedgwick as one of the forms of the pre- 

 ceding limestone, No. 1, rather than as regularly underlying it. The 

 fragments are angular and never water-worn, and appear to have been 

 recemented on the spot where they were formed. It is, therefore, 

 suggested that they may have been due to those internal movements 

 of the mass whioh produced the concretionary structure ; but the 



* Trans. Geol. Soc. LoncL, Second Series, vol. iii. p. 37. 



