Yol. 70.] OF DURHAM MAGNESIAS LIMESTONES. 251 



of the organisms in the reef to be dolomite. The loose and 

 incoherent nature of much of this material, however, indicates 

 that, in the case of some of the larger shells, the process may have 

 been a selective and partly incomplete one, the remaining calcite 

 having been dissolved out. 



The Shell- Limestone reef must have been a very porous and 

 loosely compacted structure. The matted masses of bryozoa 

 formed a sort of protective roof, preventing the dolomitic 

 sediment from filling up the interstices of the reef. The larger 

 shells are frequently seen to have been only filled up in part 

 with a dolomitic mould, consisting of decomposed fragments of the 

 smaller organisms and other reef-debris, the material having been 

 apparently injected into them in a pasty condition. It appears that 

 the dead and decomposing remains of biyozoa were as susceptible 

 •of dolomitization as are the calcareous alga? of recent atoll-reefs. 

 At the present time, in the less compacted portions of the reef 

 fossils 'are admirably preserved ; while, in the compacted portions, 

 the rock frequently becomes a hard, clean, structureless, crystalline 

 •dolomite. 



It is certainly a curious fact that large portions of the reef, 

 notably at Tunstall Hill, should have retained their calcareous 

 character, the more so, that these portions happen to be more 

 highly manganif erous. The manganese dioxide seems to be original ; 

 but it is not easy to see in what way it can have exerted any 

 influence, in retarding or arresting the dolomitization of portions 

 of the reef. 



The weathered surfaces of these calcareous Shell-Limestones 

 exhibit the utmost profusion of bryozoa and other forms, and there 

 is nothing to indicate any change in the fauna, except the presence 

 of a feAv rare forms which do not occur elsewhere. Gasteropoda, 

 especially Chitonidse, have occurred here in unusual profusion. 

 These may, however, indicate a shallowing of the water over the reef, 

 and thereby some connexion with the escape from dolomitization. 



IV. Segregation axd Dedolomitizatiox. 



The calcareous segregations observable in the Magnesian Lime- 

 stone may be relegated to two main orders. 



(1) The true spheroidal concretionary structure, chiefly de- 

 veloped on a fairly definite horizon in the lower part of the LTpper 

 Limestones, but also to a slight extent elsewhere. The opinion that 

 these structures are penecontemporaneous with the deposition of 

 the beds has been generally held since Sedgwick's time. The 

 curious spheroidal, botryoidal, radiating, and other forms taken up 

 by the calcareous material are due to distortion of the calcite- 

 <;rystals, apparently under the influence of oily or bituminous 

 organic matter. This matter may have originated through decay 

 of the organisms in the underlying adjacent Shell-Limestone reef, 

 with which the concretionary beds are rather closely associated. 



