Dr. D. Woolacott — Magnesian Limestone of Durham. 495 



mass, and shattered when the pressures developed reached the 

 compressive strength of the rock, e.g. Marsden. 1 



8. Thrust breccias, where beds have been horizontally moved along 

 a thrust-plane and shattered during the movement. These rocks in 

 some cases appear to have been cellular, segregated rocks before the 

 thrusting took place, and so were easily shattered. Such beds are 

 seen in Frenchman's Bay, on Boldon Hills, at Claxheugh, etc. 



9. Intruded breccias. Breccias which have been forced up into 

 beds above (Jean Jiveson's rock, Ryhope), 2 or down into beds beneath 

 (seen in a section near the Church, West Boldon). 



10. Missing strata, often of considerable thickness, e.g. at Clax- 

 heugh, Down Hill. 



11. Highly disturbed masses of limestone which have been moved 

 horizontally out of their original position, e.g. near Hylton Castle, 

 Claxheugh, Frenchman's Bay. 



12. At Marsden and Hendon, on the edges of the syncline beneath 

 Sunderland, the limestone has been fractured into large irregular 

 blocks without appreciable displacement. These rocks I have called 

 the " block fractured ". 



13. Slickensided surfaces are common and are usually found in all 

 exposures where thrust movements have taken place. Minute 

 slickensided surfaces have been observed in the Marl Slate at Culler- 

 coats and Claxheugh, while a horizontal slickensided surface of some 

 extent is exposed beneath the base of the reef at Claxheugh. The 

 breccia of Jean Jiveson's rock is cut by several vertical slickensided 

 planes (Photograph, PL XII, Fig. 1). 



14. Many of the fissures, breccia-fissures, and (probably also the 

 breccia-gashes) where formed by tension when the pressure was 

 relieved by movement and shattering of the strata. 



The Beeccias. 



The brecciation of the strata is one of the most pronounced features 

 of the Durham Magnesian Limestone. In the course of this paper 

 four or five types have been mentioned, and several causes have been 

 referred to as having produced these autoclastic and clastic rocks. 

 It has also been asserted that sometimes two causes may have 

 contributed to the production of some of these shattered beds. 



In a rock which over a large area has been deformed by thrusting 

 within the zone of fracture by forces of some magnitude, and which 

 has had removed from it beds generally more or less lenticular and 

 probably of considerable extent, and sometimes also of great thickness 

 (probably reaching two or three hundred feet), veins, and intercala- 

 tions of one of its original constituents, it is evident that such 

 shattering of the strata must have been brought about by these two 

 causes, and that it may sometimes be impossible to decide how far 

 only one of these was operative in producing particular breccias. 

 The cause of the brecciation is further complicated by the fact that 

 segregation and solution changes have also contributed in bringing 



1 Memoir on Marsden, p. 5 and photograph. 



2 I regard the breccia of Jean Jiveson's rock as being directly connected 

 with the thrust-plane at Hendon about half a mile to the north. 



