Litliology of Durham Magnesian Limestones. 1037 



enquiry into the composition and litliology of the Magnesian Lime- 

 stones of Durham in all their divisions and conditions of alteration. 

 More than ninety analyses were carried out, of which seventy-eight 

 are herewith presented. Several of the rocks were sliced and 

 stained with Lemberg's solution. 



Care w T as exercised, before a sample was taken, to ascertain the 

 degree of alteration, through segregation or otherwise, which the 

 rock had suffered. 



The results show r that the formation maintains, generally 

 speaking, a highly dolomitic character, with certain important 

 exceptions. Those portions which show r a calcareous composition 

 may be regarded as the result of one of three main causes : — 



(1) Original conditions of sedimentation, during- which dolomitic deposition 

 or processes of secondary dolomitization were temporarily arrested. Cal- 

 careous beds with a brachiopod fauna are extensively developed near the base 

 of the Lower Limestone in the south-western portion of the area. 



(2) Escape from secondary dolomitization. Portions of the Shell-Limestone 

 reef, notably at Tunstall Hill, from causes only partly explicable, have 

 escaped conversion into dolomite. 



(3) Calcareous segregation, penecontemporaneous with, or subsequent to, 

 deposition. 



The paper is intended to be purely a record of observed facts, and 

 no theoretical questions are raised ; but internal evidence on several 

 points is brought forward in favour of the view of direct sedimen- 

 tation of dolomite from the waters of the Permian sea. The view 

 that the bedded dolomites are the result of secondary dolomitization 

 of calcareous organisms is a very improbable one. The question 

 of the secondary dolomitization of the Shell-Limestone reef is 

 discussed. 



The dedolomitization of the formation is due to the mechanical 

 w T ashing-away of pow 7 dery dolomitic material through the inter- 

 stices of the rock. The nature of this material was investigated 

 chemically and microscopically. It results from the withdrawal of 

 interstitial calcite, both through former processes of segregation 

 and under existing conditions through the action of percolating w T atei\ 



j\b evidence of an}?- leaching-out of magnesium carbonate from 

 the rock was found. Dolomite, even in a fine state of division , 

 is almost insoluble relatively to calcite, but a question certainly 

 arises as to whether such was also the case in earlier periods, in 

 presence of saturated or supersaturated solutions of sulphates. 



The nature and distribution of the true cellular rock is discussed, 

 and modes of origin are suggested. 



Some general deductions are drawn from evidence of insoluble 

 residues. 



Finally, a summary of the general conditions of deposition of 

 the Durham Permian, from the Marl Slate upwards to the Salt 

 Measures, is given, so far as seems legitimately deducible from the 

 available facts. 



2. ' On the Occurrence of a Griant Dragon-Fly in the Radstock 

 Coal Measures.' By Herbert Bolton, M.Sc, E.R.S.E., F.Gr.S., 

 Reader in Palaeontology in the University of Bristol. 



