Frof. T. McKenny Hughes — Bursting Rock-surfaces. 511 



Wootton, and are full of the characteristic Hydrobia Chastellii, H. 

 pupa, Neritina tristis, Modiola PrestwicMi, etc. 



Borings on each bank of the Medina prove that the Hempstead 

 Beds cross the river unaffected by any fault, as also does the 

 Bembridge Limestone at Cowes. The singular rise of the Osborne 

 Beds at East Cowes, which has always been taken to indicate a fault, 

 is caused by the intersection at right angles of two undulations, which 

 produce a skew dip. Building operations at East Cowes show that 

 the Limestone falls from Osborne to the bank of the Medina, where 

 it can now be examined on the foreshore nearly opposite the 

 exposure on the left bank of the river. 



The Bembridge Marls also prove to be of considerably greater 

 thickness than was formerly thought ; they probably average about 

 120 feet. 



VIIL — Bursting Eock Surfaces. 

 By Prof. T. McKenny Hughes, M.A., F.G.S. 



THE interesting note by Mr. Strahan in the Geol. Mag. for 

 September, 1887, on explosive slickensides, reminded me of 

 some similar phenomena which were not of unfrequent occurrence 

 near Dent Head and Kibble Head in Yorkshire. 



In the limestone quarry from which the black marble of Dent is 

 procured the workmen found that, when they were quarrying the 

 lower beds and struck the rock with a pick or bar, fragments flew 

 up into the air with greater force than could be due to their blow 

 and in an unexpected direction. 



Also, when the tunnel was being made above Eibble Head, and 

 the workmen were engaged upon the bed of rock which formed the 

 floor of the tunnel, pieces used to burst off with a loud noise, so 

 that some thought they had discovered a detonating shale. 



The explanation in both these cases seemed to be that the bed 

 which was apt to shell off in that unexpected manner rested on shale 

 which yielded to the superincumbent weight on either side, and 

 produced in the tunnel, or in the quarry, where the overlying rock 

 had been removed, what would be called in a coal-mine a "creep" 

 (see Woodcut). 



Diagram showing the manner of occurrence of Bursting rock at Dent Head and 

 Kibble Head in Yorkshire, a rock, b shale ; the arrows denote the direction of 

 the pressure. 



The shale behaves as a thick fluid or viscous mass, and transmits 

 the pressure and motion. But in the cases to which I refer, a thin 

 bed of the solid rock was left above the shale. This was not com- 

 pressible, but, where, in the tunnel or in the centre of the quarry, 



