250 MR. C. T. TKECHMAKN OX THE LITHOLOGY [June I914,. 



that these thin plate -like aggregates would become eroded or 

 dissolved up again during the process. On the contrary, the edges 

 of these crystal impressions still retain a knife-like sharpness. In 

 these beds, which are very widespread, especially in the Upper and 

 Middle Series, a rapid deposition of anhydrite seems to have taken 

 place concurrently with the formation of almost pure dolomite, in 

 which the structures became embedded. 



(iii) Towards the lower parts of the thick anhydrite-mass at 

 Hartlepool, where carbonates begin to appear, first in faint streaks 

 and lower down in thicker bands, their composition was found to 

 be that of a pure dolomite. If the dolomitization were of a 

 secondary nature, one must presuppose the same process to have 

 been in action when the conditions had so changed that a thick 

 mass of anhydrite was being deposited, in order to convert every 

 fragment of carbonate into dolomite. 



(iv) Stained sections of the oolites in the Magnesian Limestone 

 indicate that in every case where the structures have remained 

 unchanged, 1 the concentrically-arranged material surrounding the 

 nuclei is practically pure dolomite. The frequent departure from 

 the dolomitic proportions in bulk analysis of many oolites is due to 

 the nuclei of the structures being of calcite, but whether this 

 calcite is in every case original or not would be hard to say. I see 

 no reason to assume that these dolomitic oolites originated otherwise 

 than in the simplest conceivable manner, through deposition round 

 nuclei moving under wave or cvrrent-action, No organic structures 

 seem to have interfered with their perfectly regular growth. 



None of these considerations would, however, preclude the 

 possibility that dolomitization of the calcareous material took place 

 either at the instant of precipitation, or during its passage through 

 the waters of the sea. 



Carbonate of lime must have been present in the waters of the 

 Permian sea, in order to supply the organisms of the reef with 

 the large quantities required by them. It is conceivable, owing to 

 the restricted distribution of the fauna, that only a portion of the 

 carbonate would be withdrawn in this manner, leaving the remainder 

 to form a direct precipitate. The conditions prevailing in the 

 Permian sea seem, then, to have brought about the direct sedimen- 

 tation of dolomite. 



At the same time, there is every reason to suppose that exten- 

 sive secondary dolomitization was taking place in the submerged 

 portions of the Bryozoan Reef, resulting in the ultimate con- 

 version of the greater portion of the Shell-Limestone reef into 

 a mass of practically pure dolomite. 2 Both analysis and micro- 

 scopical examination show the material representing the tests 



1 Both oolites and their matrix sometimes become more or less completely 

 calcified or silicified. 



- The reef in Durham extends over a distance of more than 20 miles, with 

 an average width of about 2 miles, and an original thickness of about 300 feet. 



