464 Dr. D. Woolacott — Magnesian Limestone of Durham. 



covered by the boulder-clay), is not in any way connected with the 

 cause of the formation of the concretions, but is set free as the result 

 of the alteration of these beds by the concretionary processes. 



3. The segregated limestone are also due to the crystallisation of 

 calcium carbonate from dolomitic limestones, but here the growth of 

 ■the calcium carbonate is irregular, assuming no definite forms. 1 

 They are specially characteristic of the Middle Limestone on both 

 sides of the Reef, reach a thickness of at least 90 feet, and have 

 a considerable extension in these beds. They were originally 

 dolomitic limestones of intermediate composition and probably were 

 always originally gypsiferous. During the metamorphism the entire 

 structure of the rocks has been altered, the fossils when present 

 obliterated, and the composition, texture, and colour changed. This 

 alteration of these beds had also largely taken place before the 

 thrusting (although solution and segregation of calcium carbonate 

 can also be proved to have gone on both during and after the 

 deformation of the strata). Dr. Trechmann has said that loosening 

 ■of dolomitic grains would be brought about merely by the leaching 

 out of the gypsum, and this would be so, but it appears to me 

 probable that in these rocks the segregation of the calcareous portion 

 of the rock may have taken place in sulphate solutions, and much 

 solution of the magnesium carbonate of the dolomite may have gone 

 on. The dolomitic matter found in the cavities is always in a fine 

 state of subdivision as it is in the concretionary limestone, while in 

 the Middle division of the limestones thick unaltered beds of coarse 

 granular dolomitic grains occur, and of dolomitic oolite, and the 

 question arises what was the original nature of the dolomite in 

 parts of these highly altered beds. It is again also noticeable that 

 the unsegregated dolomitic limestones from which the altered rocks 

 were formed do not contain a high percentage of interstitial calcite. 2 

 Another point of interest is whether the non -concretionary nature of 

 the calcite in these rocks is owing to the solution not having 

 contained collodial organic matter. In this connexion it is significant 

 that the concretionary limestones lie above the segregated. In the 

 latter rocks — whatever may be the way in which they were altered 

 — the calcium carbonate has segregated out, and as a result loose 

 powdery dolomitic material has been set free, which has been, and 



1 Trechmann gives the following analyses from the coast near Horden — 



Dolomite 

 Calcite . 



Insoluble residue 

 Iron or manganese 



2 An analysis of a hand specimen by the late R. C. Burton, B.Sc, of 

 fossiliferous dolomite passing into a segregated limestone from which the 

 fossils were obliterated gave — 



Dolomitic Limestone. Segregated Limestone. 

 CaCOs . . 50-39 91-92 



MgCOs • . 40-93 8-01 



In both the unaltered rocks the interstitial calcite is small, in the first case 

 •exceedingly so. 



