internal Structure of the Magnesian Limestone. 99 



because^ however important in other respects, they produce very little effect 

 upon the mineralogical character of the different systems of beds which have 

 been described. 



There are very few traces of them in Nottinghamshire and Derbyshire. In some parts of 

 Yorkshire (e. g. the escarpments on both sides of the Don, Stubbs Hill, Wentbridgc Hill and 

 the hills to the north-east of Masham, &c. &c.) they arc found in considerable abundance, but 

 in a rather obscure form, generally among the lower and more coherent beds of the deposit ; 

 while in other parts of the county we may follow the same beds through districts of many miles 

 in extent, without seeing the impress of any organized being. 



Many organic remains may also be seen on the weathered surfaces of the rocks on the coast of 

 Durham (e. g. the south end of Black rocks, Tynemouth cliff, &c. «&:c.); but incomparably the 

 most interesting specimens are found in some shelly coralline masses which occupy the central 

 portion of the deposit in the same county. 



The most northern point at -which I have seen them (although they probably extend still 

 further), is Ilumbleton quarry, a well known locality on the Durham road, about two miles from 

 Sunderland. There, they occupy the top of the escarpment, resting upon a set of thin brown 

 cellular beds. Here and there, they are tinged with ochreous stains; but their prevailing colours 

 are light grey or yellowish white. Their texture is in part earthy and rubbly ; other parts are 

 hard, subcrystalline, and porous, but not cellular ; more rarely they pass into beautiful, hard, 

 crystalline masses, like the coral rag of the middle oolite. 



With all these varieties of structure, and with local modifications, they may be followed into 

 Tunstal Hill ; and thence, ranging nearly south, they pass over the hills between Dalton.le-Dale 

 and Easington. Their further range is disguised by diluvium. 



In this way have shelly beds been traced six or eight miles in the direction 

 of their range through the very heart of the formation, and they may be pro- 

 longed considerably further. As they contain a fine suite of organic remains, 

 (e. g. corals, encrinital stems, casts of univalves, and several species of the 

 genera Producta, Area, Terebratula, and Spirifer, &c. &c.), they throw a 

 most important light upon the natural history of the formation to which they 

 are subordinate : and it is chiefly by their help that we arrive at the following 

 important conclusion ; viz. that the magnesian limestone, notwithstanding its 

 unconformable position, is, in zoological characters, more nearly allied to the 

 carboniferous order than to the calcareous formations which are superior to 

 the new red sandstone. 



From what has been here stated, combined with many obscure indications 

 of organic remains in other parts of the formation, it is probable that marine 

 shells existed in great abundance during the whole time that the magnesian 

 limestone was deposited ; and that they have in a great measure disa})peared, 

 because they did not find in many portions of it a matrix proper for their pre- 

 servation. In the pulverulent beds they could not undergo petrifaction or 

 form casts, and would, therefore, almost of necessity, be absorbed and carried 



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