254 MB. C. T. TKECHMANN ON THE LITHOLOGT [June 1914, 



No. L, merely by the leaching-out of the gypsum which im- 

 pregnates it. 



Incoherent powdery material, of the nature just described, is 

 apparent in nearly every exposure of the Magnesian Limestone in 

 Durham. When developed on an extensive scale, however, it is 

 always closely associated with calcareous segregations. It may be 

 concluded, then, that the main factor concerned in the 

 liberation of the powdery dolomite, is the withdrawal 

 of interstitial calcite from the rock through former 

 processes of segregation and, under present conditions, 

 through the solvent action of percolating water. 



The greater solubility of the calcite in contrast with the relative 

 insolubility of the dolomite under ordinary conditions, is clearly 

 indicated by several observable facts : — 



(a) Geodes lined with large secondary calcites l are very frequent in friable 

 dolomitic rocks. The cavities are apparently contraction- effects, and the 

 presence of such calcite-lined cavities is a very certain sign of alteration in 

 the rock. 



(b) Large doubly-terminated calcite-twins occur embedded in the dolomitic 

 matrix of the concretionary structures at Fulwell, Byer's Quarry, etc. 



(c) In some compact dolomites in the Lower Limestone, thin calcite- veins 

 traversing the rock are seen on weathered surfaces as channels dissolving out 

 more rapidly than the dolomite which encloses them. 



(d) The instance of the Knaresborough ' dropping well,' where water passing 

 through a porous dolomitic rock coats objects with a calcareous encrustation. 

 (This instance need, however, scarcely be mentioned ; also, it is not in 

 Durham.) 



The conditions most favourable to the production of powdery 

 material are those in which a relatively- small quantity of calcite 

 exists disseminated through a highly-dolomitic rock, in which 

 it acts as a binding medium to a large quantity of dolomite- 

 grains. Such conditions are present, to a great extent, in all 

 divisions of the Magnesian Limestone in Durham. 3 



On the other hand, the condition which has allowed the escape 

 of certain patches of the formation from the general process of 

 segregation, collapse, and dolomite-removal which has overtaken 

 so many beds, has been in many cases the local presence of an 

 exceptionally large proportion of dolomite. In other words, on 

 removal of the small quantity of interstitial calcite, the dolomite- 

 grains still remain so closely interlocked that the tendency to 

 collapse and become washed down is overcome. In such cases, the 

 mass of incoherent dolomite-grains may still retain the impressions 

 of enclosed fossils more or less faithfully preserved. 



The bedded rocks still enclosing casts of Astarte vallisneriana 

 in Castle Eden Dene are theoretically pure dolomites. In certain 



1 Dolomite never occurs thus. I have found nests of small dolomite-rhombs 

 loosely filling rounded cavities in some beds, having seemingly grown in a 

 matrix of gypsum now dissolved away. 



3 A similar feature has been observed by Mr. E. E. L. Dixon in the case of 

 Carboniferous dolomites, Q. J. G. S. vol. lxvii (1911) p. 314. 



