252 ME. C. T. TEECHMANlSr C»" THE LITHOLOGT [June 1914,. 



The calcite and dolomite in these concretions and their matrix 

 are, as a rule, less completeh' separated than in the concretions 

 of the next order. 



Fossils are generally well preserved in or upon the calcareous 

 segregations, but they exist only as empty casts in the adjacent 

 dolomitic beds. 



(2) Irregular segregations, presumably contemporaneous with, 

 and subsequent to, the removal of sulphates from the rock. Large 

 tracts of Middle and Upper bedded rocks are changed in this way. 



Fossils are in most cases entirely obliterated^in the calcareous 

 portions through recrystallization, in the dolomitic portions through 

 the washing-down and degradation of the loose powdery material. 



The fine powdery matrix 1 is very similar in both types of con- 

 cretions. Analysis indicates the proportion of dolomite in it to 

 range from about 87 to 90 per cent, in the first type, and from 

 about 95 to 98 per cent, in the second type. In the first type 

 the material is homogeneous throughout ; but in the second 

 type the larger residue remaining after riddling is more calcareous, 

 approaching the composition of the segregated portions of the 

 rock. 



A microscopic examination of the small residue, which remains 

 after riddling the very fine material through a sieve of 240 meshes 

 to the linear inch, shows the nature of the powdery matrix of 

 the concretions to be as follows : — 



The greater part consists of aggTegates of minute crystalline dolomite-grains ; 

 in many cases, they occur as clear, colourless or yellowish, rhombic crystals. 

 Some of the aggTegates are coated and cemented together with a dark dendritic 

 encrustation of manganese dioxide. Mixed with these occur much larger 

 clear calcite -grains, often in the form of doubly-terminated scalenohedral 

 crystals. Some are lying clear in the mass, singly or in aggregates. Others 

 are coated with, and partly hidden by, the dolomitic grains, as though they 

 were in the act of rejecting these during growth. Siliceous and other residues 

 lie loose in the mass. 



No analyses seem to have been made of the gypsiferous beds, 

 bored through in the Seaton-Carew or other similar borings, and 

 all the carbonates that I examined from a boring under a mass. 

 of anhydrite at Hartlepool were dolomites. The question as to 

 Avhether in former periods, in the presence of saturated or super- 

 saturated solutions of calcium sulphate, the dolomite was as stable 

 as it is under present conditions, or whether gypsum in any form 

 can have assisted the segregatory migration of calcareous particles, 

 remains a problematical question. 2 



1 The German term Dolomitgruss or Dolomitasche is applied to a 

 similar material consisting largely of minute dolomite-rhombs. I have seen 

 it in cavities in rauchwacke in the Gera district, but never approaching the 

 extent to which it is developed in Durham. German investigators seem to 

 regard it as a residue left after solution of gypsum. 



2 Calcareous replacement of dolomite to an abnormal extent is often visible 

 in the vicinity of faults or other planes of movement, and would seem to- 



