Dr. D. Woolacott — Magnesian Limestone of Durham. 465 



is being, mechanically removed. The rock as a whole is thus finally 

 losing the greater percentage of its magnesium-content. These 

 segregated rocks are often broken up without much displacement, 

 being generally compactly recemented, thus forming one class of 

 pseudo-breccias. 



In the case of both the concretionary and segregated rocks the 

 solutions must have been saturated so that solution and precipitation 

 were going on at the same time, otherwise the stability of the rocks 

 would have been affected and the strata broken up. The water 

 which filled every cavity and pore would also help to prevent the 

 collapse of the rocks at the time the structures were forming. The 

 pseudo-brecciation, brecciation, and disturbances of the strata can 

 generally be proved to have taken place after the structures were 

 developed in them. 



4. The other type of cellular limestone from which dolomite has 

 been removed is the rock I have named the "Fractured-cellular". 

 Here again alteration of dolomitic limestone into a calcareous rock 

 has taken place along fractures and lines (apparently necessitating 

 solution and segregation changes similar in nature to those already 

 discussed) and the loose dolomitic rock is then mechanically removed. 

 This type is well exposed along the coast to the south of Seaham 

 Harbour. 



5. Between Frenchman's Bay and Marsden dolomitic rocks have 

 been brecciated along a thrust plane, and subsequently bound 

 together by a calcareous cement. This material may have been 

 derived by solution from the dolomitic powder produced at the 

 time of thrusting, when there would be both high temperature and 

 increased pressure, or may have been segregated out of the fragments, 

 or may have been brought in from the rocks above. At the present 

 day the dolomitic limestone of the breccia is loose and powdery, and 

 is being removed mechanically. The original dolomitic breccia thus 

 becomes a highly, and very coarsely cellular, calcareous rock, i.e. 

 a " negative breccia ". 



The evidence for these alterations in the magnesian limestone 

 «an only be studied in the field, and there is no doubt that the more 

 these rocks are observed, the more striking do the extensive changes 

 become. The theories advanced in this paper are not put forward in 

 any dogmatic manner, but merely as an attempt to explain them. 

 (To be continued.) 

 Addendum. 



1. Where statements are made regarding the composition of rocks and 

 analyses were not available they have always been analysed, generally by post- 

 graduate students of Armstrong College. 



2. In connexion with solution changes in the magnesium limestone, it is 

 perhaps worthy of note that Sunderland Water contains on the average — 



per 100,000. Grains per gallon. 



CaO . . 10-99 • 7-69 



MgO . . 6-38 4-47 



SO-3 . . 3-83 2-68 



C0 2 . . 12-42 8-69 



As a very large and busy manufacturing district obtains all its water from this 



formation, large quantities of both calcium and magnesium must yearly be 



dissolved out of the magnesian limestone. 



DECADE VI. — VOL. VI. — NO. X. 30 



