is*. Coohsey Burton — Yellow Sands and Marl. 



305 



of the neighbouring region. Now when the thrust 1 acted on the 

 Permian rocks of this area the Magnesian Limestone and Marl Slate 

 were much disturbed, brecciated, contorted, and pinched out in various 

 ways ; fissures were produced, much shearing and slickensiding took 

 place, and opportunities for the circulation of water were greatly 

 increased. At Trow Rocks the thrust-plane can be seen at the north 

 end of the quarry dipping to the south, and we find brecciated 

 Magnesian Limestone overlying unbrecciated beds beneath the thrust- 

 plane. The Marl Slate and 'Yellow Sands' cannot be seen in situ, 

 being below tide-mark, and the effect of the thrust on those beds 

 cannot be exactly determined at this point. The Marl Slate is 

 a relatively impermeable bed, and would, no doubt, in its normal 



Fig. 4. Section of lenticular hollow in the Magnesian Limestone, containing 

 Yellow Sands. The Marl beds are 2 inches to 3 inches thick ; total 

 thickness of lenticular bed, 4 feet ; total length of lenticular bed, 20 feet. 



condition prevent the circulation of much water through it. The 

 Marl Slate at Trow Rocks must therefore have been so fractured and 

 possibly brecciated by the thrusting movements that water could 

 freely percolate through it and through the joints. In many sinkings 

 made in the North of England Permian water in large quantity has 

 been struck, and in its rise from below has carried up large amounts 

 of ' Yellow Sands ' ; it is thus that the lenticular beds of sand and 

 marl in the Magnesian Limestone have been formed. Subsequent to 

 the brecciation of the rocks percolating water has excavated hollows 

 in the breccia ; underground water has accumulated to heights of 

 at least 50 to 60 feet above the ' Yellow Sands', and, being probably 

 subject to rises and falls for some unknown reasons, has carried up 

 with it quantities of the loose sand and deposited it in the above- 

 mentioned hollows and caverns, and thus formed secondary beds of 

 ' Yellow Sands '. 



In all cases the beds of ' Yellow Sands ' and marl conform to the 

 irregular surfaces of the limestone and also quite fill the hollows, 



1 Dr. Woolacott, " A case of Thrust and Crush-brecciation in the Magnesian 

 Limestone of County Durham " : Univ. Durham Phil. Soc, Memoir No. 1. 

 DECADE V. — VOL. VIII. — NO. VII. 20 



