452 C. Davison — British Earthquakes of 1890. 



On the same day, at about 4h., a third shock is said to have been 

 felt, at Wetherby ; but, in the absence of any further information, 

 its occurrence must be regarded as doubtful. 



Origin of the Shocks. — In the immediate neighbourhood of the 

 epicentrum, there appears to be no fault with which these earth- 

 quakes may be connected : at any rate, none is marked upon the 

 Survey map. The evidence given above, slight as it is, seems to me 

 also rather to oppose than to favour the view that they were fault- 

 formed shocks. 



Somewhat similar shocks, though disturbing smaller areas, are of 

 frequent occurrence at, and in the neighbourhood of, Sunderland. 

 These have been studied and discussed by Prof. G. A. Lebour in an 

 admirable paper "On the Breccia-gashes of the Durham Coast, etc." ^ 

 The magnesian limestone at Sunderland is about 400 feet thick. 

 It contains numerous caverns, many of them naturally formed, some 

 probably artificial owing to the withdrawal of water by the local 

 water company. Fragments of rock must frequently fall from the 

 roofs of these caverns, and it is to the concussions produced by them 

 that Prof. Lebour, with very good reason I believe, attributes the 

 Sunderland shocks. That such falls have frequently taken place 

 in former times is obvious from the constitution of the numerous 

 " breccia-gashes " which may be seen along the coast of Durham. 

 That they are probably continuing at the present time is shown by 

 evidence which has been obtained since the publication of Prof. 

 Lebour's papers. The Sunderland shocks are especially numerous 

 at certain parts along the banks of the Hendon Burn, a small 

 stream about a mile south of the Eiver Wear. The Sunderland 

 and Ryhope Eoad crosses the stream nearly at right angles, and, 

 for a distance of about 400 yards on the north side, and about 

 150 yards on the south side of the stream, it has been found that 

 the road has recently subsided. The Ordnance Survey levelling of 

 the road was carried out in 1857, and in 1887 the levelling was 

 repeated by a well-qualified surveyor at the instance of Mr. T. W- 

 Backhouse, of Sunderland. The average subsidence of the road 

 over the distance mentioned was found to be 1 foot 10 inches in the 

 thirty years ; at the stream itself the subsidence was 2 feet 4 inches. 

 A large part of this subsidence seems to have taken place towards 

 the end of this period ; for, in four years, 1883-87, Mr. Backhouse 

 found by observations of a distant chimney, that his observatory, 

 which is close to the Eyhope road, had been lowered by 1 foot 4 inches, 

 and it is worthy of notice that just about this time the slight shocks 

 became especially frequent. 



The part of the disturbed area surrounding the epicentrum of the 

 first of the Yorkshire shocks consists also of magnesian limestone, 

 though its thickness is less than at Sunderland.^ The depth of the 



^ Trans, of the N. of Engl. Inst, of Mining Eng. (Newcastle-upon-Tyne), 

 vol. 33, 1884, pp. 165-174. See also a paper by the same author, " On Some Recent 

 Earthquakes on the Durham Coast and their probable cause," Geol. Mag. (1885), 

 Dec. 3, Vol. II. pp. 513-515. 



2 In the neighbourhood of Tadcaster, the thickness of the Permian formation 

 (Upper Marls to Lower Magnesian Limestone, inclusive) is 300 feet (Mem. Geol. 

 Surv., Explanation of Quarter-sheet 93 S.W.). 



