Vol. 70.] OF DURHAM MAGXESIAX LIMESTONES. 255 



dolomitic fragments among segregated calcareous beds in Haswell 

 Quarries, Byeris Quarry, and elsewhere, the rock enclosing perfect 

 impressions of shells or even plant-remains is excessively friable. 

 When the material is freshly taken from the quarry, the capillarity 

 of the enclosed water holds the grains together ; but. on drying, the 

 mass crumbles to powder at a touch. 



The Flexible Limestone of Marsden (No. XX) consists of minute, 

 loosely-compacted dolomite -grains with siliceous and argillaceous 

 residues, and owes its flexibility to movement between the grains. 



On a more extensive scale, many of the highly- dolomitic upper- 

 most beds (Hartlepool and Roker Series) have escaped alteration. 

 The quantity of interstitial calcite is so small that very little 

 segregation can take place, and the resulting disturbance of the 

 dolomite is correspondingly slight. 



In a sample of Lower Limestone, however, the slight segregation 

 or concentration of calcite, indicated in analysis by a difference of 

 about 05 per cent., suffices to release a considerable quantity of 

 dolomite from a nearly dolomitic rock. This specimen (Nos. LXII 

 & LXIII) is from a depth of 725 feet in Blackhall Sinking. 

 The removal of such powdery dolomite has caused much of the 

 nodular structure and internal bending seen in the Lower Lime- 

 stones at the surface. 



The segregated Shell-Limestone at Easington (Nos. XXXI— 

 XXXVI) is of interest, because so numerous are the fossils that, 

 neither in the calcareous segregations, nor in the powdery material, 

 are they completely obliterated — showing that both these materials 

 have resulted from a change in the Shell-Limestone similar to that 

 adjacent to them. 



Careful sampling and analysis, and reduction of the results to 

 terms of calcite and dolomite, indicate that the removal in one 

 case of about 2 per cent, of calcite from the rock, in another 

 case apparently the mere recrystallization of the interstitial calcite 

 in the powdery mass, suffices to bring about the collapse of the 

 dolomite -grains, with an all but complete obliteration of the fossils. 



It is often difficult to form an opinion regarding the original 

 composition of several now highly- segregated calcareous beds. In 

 some parts of the concretionary series, in Byer's Quarry for instance, 

 segregation has taken place so evenly along the bedding-planes 

 .and the powdery dolomite has been removed so gradually, that 

 very little brecciation has resulted from the settling-down of the 

 calcareous material. In such cases, it is easy to assume a highly- 

 calcareous original nature for the rock ; but the presence of 

 occasional remaining fragments of uncollapsed dolomite-powder 

 retaining perfect fossil-impressions suggests a different inference. 



The original dolomitic nature of such beds probably approxi- 

 mated to that of adjacent beds where the development of 

 concretionary structures has been arrested, as noticed by Prof. E. 

 J. Garwood, 1 by the marly and sandy impurities present in the rock. 



1 ' Origin of Concretions in the Magnesian Limestone of Durham ' Geol. 

 Mag. dec. 3, vol. viii (1891) p. 435. 



