514 Fvof. Lehour — Recent Earth qualies on Coast of Durham. 



time the shocks having continued more or less continuonsl}'', and 

 evidence of all kinds with regard to them having accumulated, I 

 wish to lay my more mature views on the subject before Section C, 

 in the hope that members in discussing them may help to elicit the 

 truth. 



Sunderland stands upon the Permian Magnesian Limestone, 

 There is from 300 to 400 feet of this rock beneath the town. This 

 rock is riddled with cavities of every size and shape. ^Jlie smaller 

 ones give a vesicular aspect to the stone in many places, but the 

 larger ones are often true caverns due to the combined action of 

 mechanical and chemical agencies. Many of them may be accounted 

 for by noting how frequently masses both large and small and of 

 all shapes of soft pulverulent matter occur in the midst of the most 

 compact and hard portions of the limestone. How easily such soft 

 incoherent earthy rock or " Marl " as it is called locally, can be 

 removed by the merest percolation of rain-water, needs no proof, and 

 that caverns would result and have resulted from such removal is 

 clear. This action is indeed chiefly mechanical, but there is also 

 going on at the same time a very considerable destruction or removal 

 of rock by the ordinary chemical action of rain-water on limestone. 

 I have shown elsewhere that every thousand gallons of Sunderland 

 water pumped up, and ultimately thrown into the sea, represents one 

 pound of stone abstracted. In each year the Water Company robs 

 the Magnesian Limestone in this manner of about forty cubic yards 

 of rock, and of course much more is carried off annually by natural 

 channels. How large some of the cavities are which form water- 

 cisterns in this rock may be gathered from the fact that when in 

 "sinking a shaft at "Whitburn Colliery in 1874, one of them was 

 unfortunately tapped, it yielded 11,612 gallons of water per minute 

 for a month. 



The rock then immediately underlying Sunderland is a mass of 

 calcareous stone, mostly hard and compact, but cellular in places 

 and earthy and friable in others, often cavernous on a large scale, 

 full of water, and through its action continually parting with its sub- 

 stance, and thus enlarging the cavities within it. 



Under conditions such as these, it follows necessarily that the 

 vaults of cavities must from time to time give way, and in col- 

 lapsing, produce concussions accompanied by noise, but limited in 

 the area over which their effects would be felt. In short it seems to 

 me that we have in such natural stone-falls at moderate depths a 

 sufficient explanation of the Sunderland earth-shocks. 



In the paper before alluded to I pointed out that this theory 

 explains equally well all the facts connected with the singular 

 fissures full of breccia ("breccia-gashes") which are common in the 

 Magnesian Limestone of Durham, and have been a standing puzzle 

 to Lyell, Sedgwick and all the geologists who have published 

 accounts of the magnificent sections exhibited along the coast between 

 South Shields and Sunderland. 



Quite recently very similar shocks have been felt, as I am in- 

 formed, in the neighbourhood of Middlesborough, where it is pro- 



