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



about an unstable condition of the strata, and that contemporaneous 

 brecciation also occurs. 



The breccias may be classified as follows: — 



1. Contemporaneous Breccias. — In the Lower Limestone as recorded 

 by Trechmann and myself, incipient brecciation occurs. 1 The rock 

 is minutely fractured without any appreciable displacement, a 

 feature most probably due to shrinkage of the rock on drying. 



2. Clastic Breccia. — The interbedded fossiliferous breccia- con- 

 glomerate of Blackhall Rocks and Hesleden Dene, i.e. the Vor-reef, is 

 an example of this type (Photograph, PI. XII, Fig. 2). 



3. Pseudo-breccias. — (a) Some dolomitic rocks in the Magnesian 

 Limestone assume a brecciated appearance due to the patchy 

 development of fine yellow dolomite among the crystalline calcite. 2 

 These are true pseudo-breccias, and have been brought about by 

 segregation processes. 



(b) Some of the rocks which were called pseudo-breccias by King 3 

 are segregated rocks which have broken up, (1) by mere collapse 

 due to their cellular nature when the sulphates were removed and 

 the segregation processes complete, (2) by the letting down of these 

 beds due to the - removal of the sulphates, 4 and (3) by shattering 

 produced by movement at the time of thrusting. 5 



4. Autoclastic Breccias. — Breccias directly due to thrusting include 

 (a) crush breccias, (b) thrust breccias, (c) brecciated folds, and (d) rocks 

 which have been broken up after having had compression-jointing 

 impressed on them. 6 



5. Solution Breccias. — Breccias due to the collapse of the strata 

 caused by the removal of beds of anhydrite or gypsum.' 



6. The gypsiferous breccias occurring in the Warren Cement 

 Works boring are probably due to the breaking up of the rock caused 

 by the expansion produced by the alteration of anhydrite into 

 gypsum. 



7. Vein-dolomitized Breccias. — The coarse dolomitic breccias 

 exposed in the bluestones of Ilaisby Hill Quarry (calcite 98-9 

 per cent) are due to the alteration of these rocks by magnesium, 

 copper, and manganese solutions. 



8. Breccias filling breccia-fissures and breccia-gashes. These are 

 well exposed in the Marsden district. The vertical fissures filled 

 with coarse breccia are sometimes 40 and 50 feet wide, and large 

 blocks have fallen considerable distances into them. 



1 Woolacott, Geol. Mag., March, 1919, pp. 164-5. 



2 Similar rocks have been noticed by L. M. Parsons in Carboniferous 

 dolomites. " Dolomitization and Leicestershire Dolomites": Geol. Mag., 

 1918, p. 250. 



3 King, Monograph of Permian Fossils, Pal. Soc, 1849. 



4 Broken up segregated rocks of the first two types occur on the eastern 

 flanks of the reef. Vertical slickensided surfaces produced by the slipping 

 down of these beds have been noticed by Trechmann. 



5 Shattered rocks of this type occur in the Marsden area and in the 

 Claxheugh and Boldon Hills districts. 



6 These are all best developed in the Marsden district. 



7 The very coarse breccias south of Seaham Harbour where brecciation 

 associated with faulting is extensively developed are probably the best example 

 of this type. 



