1814.] Northumberland, Durham, &c* 41 f 



lustre glimmering, from a few small crystals of calcareous spar, or 

 bitter spar, interspersed : specific gravity 2*637 ; but as the stone is 

 very porous, that was probably below the truth. Its composition 

 was as follows : — 



Carbonate of lime 51 *50 



Carbonate of magnesia 44*84 



Insoluble matter 1 *G0 



Loss 2-06 



100-00 



I find the carbonate of magnesia in these lime-stones composed 

 of one atom caroonic acid and one atom lime. This ariety dis- 

 solves in acids as rapidly as pure carbonate of lime. 



I cannot find that any of the shells which occur in this rock have- 

 been hitherto described by conchologists. None of them are kno»vn 

 to exist in a recent state upon our coasts. E 'hat they are sea 

 shells I consider as demonstrated by the impiessions of flustras, 

 which are visible in different parts of the stone, and by casts of 

 corralines, which are not uncommon in it. 



The most curious shell has some resemblance to a shell observed 

 by Martin in the magnesian lime-stone of Bredon, in Derbyshire, 

 and probably constituting a species of the same genus. Mr. Sowerhy 

 has, in his Mineral Conchology, Tables (»8 and fJi). given figures of 

 various shells allied to it, most of which he got from Dr. Fleming, 

 of Flisk, Fifeshire. Mr. Sowerhy has constituted these shells into 

 a new genus, to which he has given the name of producta. Our 

 shell, then, seems to be a species of producta. Mr. Sowerhy con- 

 ceives that he can distinguish no fewer than casts of five species of 

 producta in the specimen which I brought from Humbleton Hill. 



Another very perfect cast appears to he a new species of pecten. 

 There is a small cast of a shell which Mr. Sowerhy thinks like the 

 terebratula subrotunda. These, with various casts of encriui, are 

 all the shells that 1 have been able to observe. From these shells it 

 is obvious that this magnesian lime-stone was deposited at the. 

 bottom of the sea, and that, though the most recent of all the beds 

 existing in the county of Durham, it must be of very ancient date, 

 since the deposiiion must have. taken place before the existence of 

 the shell fish which at present live upon our coast. 



3. Flexibk I a mc- stone from Marsdon Rock, on the Sea Shore Jive 

 Mitts north of Sunderland. 



I did not visit this rock myself; but got the specimens of it, from 

 which my examination was made, from Di. K<id Clanny, of Sun- 

 derlaod, to whose mineratogical zeal 1 am indebted, Dot only foe 

 my knowledge of the Axis tea Of? of this curious rock, but lor lie 

 opportunity of examining all the spots which attracted my attention 

 in the neighbourhood of Sunderland* Ail the specimens which I 



Vol. IV. N° VI. 2 D 



