1866.] KING AND EOWNET " EOZOONAL ROCK." 211 



ence has already been made to the cupriferous porpyhry of Canada, 

 some zeolitic traps, and the olivine-lava of the Ponza Islands. We 

 shall next refer to a remarkable deposit of sedimentary origin. 



Few rocks in the entire series of geological formations present any- 

 thing like the singular configurations which characterize the Permian 

 Magnesian Limestone of the neighbourhood of Sunderland*. In 

 other parts of Durham, as along the coast from Ryhope to Hartle- 

 pool southward, and from Marsden to near South Shields north- 

 ward, this rock has the ordinary lithological structure — earthy, or 

 compact, occasionally oolitic, &c. ; but in many places on both sides 

 of the mouth of the Wear, it is more or less crystalline ; and 

 where this is the case, there are often exhibited the most remarkable 

 forms — honey- combed, discoidal, mammillated, and hemispherical, 

 coralloidal, stalactitic, reniform, globular, and botryoidal. Yery 

 frequently the amorphous and the crystalline varieties are intimately 

 associated, the former serving as a matrix for the latter. Thus 

 constituted, the rock often displays the most imusual appearances, 

 especially where the amorphous kind is weathered out — resembling, 

 as the case may be, massive courses of ruined masonry, reefs of 

 petrified coral, or beds of brown-rusted shot and cannon-balls. 



Like most sedimentary rocks, the Primary especially, the one 

 under consideration has its beds often very much and continuously 

 divided by joints, varying extremely in their distance from one 

 another. The beds, in consequence of this divisional structure and 

 the frequency of depositional partings, often occur as if consisting of 

 huge quasi-rhombohedral blocks. And it frequently happens that 

 the crystalline forms, wherever they are numerous in such masses, 

 are seen to strike indifferently from the planes of jointing and the 

 planes of bedding : hence it is, that dark-brown branching crystalline 

 coralloids (imbedded in a lighter- coloured matrix) may be fre- 

 quently observed striking inwardly from every face of a block. 



Where the beds are not divided by jointing, large and small 

 mammiUary and stalactitic forms may often be seen shooting off, 

 both upwards and downwards, base to base, from two adjacent 

 planes of bedding, the law of gravitation having been altogether 

 discarded by the mysterious agent which produced them. 



Erom various analyses of the rock, it has been ascertained that 

 the configurations are composed of carbonate of lime, and that the 

 matrix consists (as is also the case with the associated ordinary lime- 

 stones) of carbonate of lime and magnesia. It is therefore highly 

 probable that the absence of the magnesian constituent in the con- 

 figurations is not an original condition, but the result of some kind 

 of segregative action: and it would seem that the agent which 

 produced this result had gained admission into the very heart of the 

 beds through their joints and depositional partings, indifferently 



* These configurations Verel first described by Professor Sedgwick, in his 

 classical memoir " On tlie Geological Relations and Internal Structure of the 

 Magnesian Limestone," &c., published in the ' Transactions of the Geological 

 Society of London,' 2nd ser. vol. iii. 1826-1828. They have also been described 

 by one of us in the " Introduction " to the ' Monograph of the Permian Fossils 

 of England,' 1850, 



