4-6 PRELIMINARY NOTE ON A CASE OF THRUST AND 



Preliminary Note on a Case of Thrust and Crush-hrecciation 

 in the Magnesian Limestone, Co. Durham. By David 

 WooLACOTT, D.Sc, F.G.S., Armstrong College, New- 

 castle-upon-Tyne. 



Along the coast from South Shields to Marsden the 

 Lower, Middle and Upper Magnesian Limestone are ex- 

 posed. Although the beds have been much disturbed, yet 

 on passing southwards from the Trow Rocks to the south end 

 of Marsden Bay, these three divisions of the Permian can be 

 followed in regular sequence, with a general low dip of a few 

 degrees to the south. The Lower division is a brownish- 

 yellow, regularly-bedded rock, the base of which is not seen, 

 but some 30 feet are exposed. At the Trow Rocks and in 

 Frenchman's Bay its lower beds are not disturbed, but the top 

 layers are considerably so, being fractured, brecciated, tilted- 

 up and laterally displaced. Resting on this irregular base is 

 a mass of white brecciated limestone reaching a thickness of 

 some 50 feet. The pieces forming this bed vary in size from 

 the smallest dimensions to blocks several feet across, and are 

 generally firmly cemented together. Sometimes the fragments 

 have been dissolved out, and the rock has become cellular in 

 consequence. In some places the original major bedding- 

 planes can be observed, while in other parts the thin lamina- 

 tions are traceable. The junction with the Lower Limestone 

 is a thrust-plane, which, however, does not always exactly 

 coincide with the line separating the two beds, but is seldom 

 more than a few feet from it. The direction of the thrust has 

 here obvious)}^ been from the north — the brecciation of the 

 Brecciated Limestone as well as the disturbance of the upper 

 layers of the Lower Limestone being due to it. 



The Brecciated Beds can be followed along the coast 

 from the Trow Rocks to the north end of Marsden Bay. 

 The top, which is seen at the latter place, is very irregular 

 and hummocky. Above it occurs a soft, powdery, yellow 



