180 Reports & Proceedings— Geological Society of London. 



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 was exercised, before a sample was taken, to ascertain the 

 degree of alteration, through segregation or otherwise, which the rock 

 had suffered. 



The results show that the formation maintains, generally speaking, 

 a highly dolomitic character, with certain important exceptions. 

 Those portions which show 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. Calcareous 

 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 sedimentation 

 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 

 washing-away of powdery dolomitic material through the interstices 

 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 water. 



No evidence of any 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 

 Avhether 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 are 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 Giant Dragon-fly in the Iladstock 

 Coal-measures." By Herbert Bolton, M.Sc, F.E.S.E., F.G.S., 

 Header in Palaeontology in the University of Bristol. 



The writer describes the structure of a wing-fragment found some 

 vears ago upon the Tyning waste-heap at Radstock Colliery (Somerset), 

 by Dr. E. A. Newell Arber, F.G.S. 



