258 ME. C. T. TEECHMANN ON THE LITHOLOGT [June 1914, 



crystals, occurring in this and other residues, can scarcely be 

 detrital. 



The absence of residue in the anhydrite l points to the view that 

 "this substance originated in a manner distinct from the rest of the 

 material deposited from the sea- water. 



At certain periods, it seems, a deposit of siliceous material took 

 place. Silica, in the form of compact or friable nodules of chert, 

 occurs in several beds, but chiefly in the Middle bedded rocks on the 

 •eastern side of the reef. In many cases, these nodules merge with 

 every gradation into the surrounding dolomite ; in others, on removal 

 of the dolomite, they occur as mere angular fragments embedded in 

 recemented calcareous breccias. 



The increase of siliceous residue found, in several powdery 

 dolomites (Nos. XXXIII & XXXV), above that in the parent 

 rock whence they were derived, seems to indicate that the change 

 was sometimes accompanied by a slight concentration of silica, 

 tygether with a decomposition of the micaceous, felspathic, and 

 pj^ritous portions of the residue. 



As a rule, the Shell-Limestone, in common with reef -accumula- 

 tions, is very deficient in residue — sufficient evidence of the absence 

 of silica- secreting organisms. Generally it contains a larger pro- 

 portion of heavy minerals and a smaller quantity of quartz than 

 most of the bedded rocks. A typical Shell-Limestone residue is 

 that of No. XXXI : it contains occasional large idiomorphic 

 quartz-grains; large flakes of a white mica, showing consider- 

 able decomposition ; pyrite, with ferruginous material, frequently 

 pseudomorphic after it, resulting from its decomposition ; tour- 

 maline and zircon-grains ; and occasional small cubical fluorites. 



That found in the adjacent powder derived from it contains 

 irregular quartz-grains. Hardly any mica, or pyrite, or ferruginous 

 minerals occur ; but tourmalines and zircons are present. 



The general scantiness of residue that could be regarded as 

 detitral suggests, either the existence of arid conditions over the 

 adjacent land, or that the drainage into this part of the Permian 

 sea did not affect the cla}^ and sandy Carboniferous strata 

 presumably exposed round its margins. This is the more noticeable, 

 seeing that plant-remains 3 occur in all divisions of the Magnesian 

 Limestone (except in the reef), where the beds are sufficiently un- 

 altered to allow of their preservation. 



Regarding detrital residues, an examination of the residues in 

 relation to the dolomitic nature of the more littoral beds of York- 

 shire and Nottinghamshire would be interesting. It is evident in 

 Durham that in the Lower Limestones the residues appreciably 

 increase as we pass westwards and approach the old shore-line. 



1 The conditions of deposition of anhydrite from solution were found by 

 Van 't Hoff to be of a strictly limited nature. They suggest the formation of 

 anhydrite in the upper, concentrated, and heated layers of the sea in seasons 

 of exceptional warmth and desiccation. 



2 These are presumably drifted, although the twigs and fruit of Ullmannia, 

 so plentiful in some beds, might be wind-borne. 



