88 Johnx: S/icJfichrs Trouo/i Fun//. 



of the machinery by which faultiii'^ and disturbance were pro- 

 duced, that we can offer no explanation of the way the wrench 

 was caused. " 



It sliould not be forgotton that when the survey was made, 

 Structural Geolog^y had not received the attention that has been 

 devoted to it during- recent years. 



To beg-in with, it mig-ht be mentioned that there has been 

 no wrenching- away and twisting- round throug-h an angfle of 

 90" in the area we are discussing-, so we are spared the trouble 

 of investig-ating- the mechanism necessary to produce such 

 movements. The Yorkshire coal-field is sing-ularly free from 

 evidences of tang-ential stresses and reversed faults, and over- 

 thrusts are almost absent ; such trifling- examples as may be 

 pointed out are local in character, and only due to the efforts of 

 wedg-e-shaped rock masses to accomodate themselves to new 

 positions while underg-oing subsidence. The disturbances that 

 have determined the tectonic features of the basin, are to be 

 included in the g-roup of Gravity Faults. Gravitational stresses 

 are the only ones that need consideration, and Normal Faulting^ 

 is the rule. 



Normal faults, as. is well known, hade to the downthrow 

 side, and from their very nature involve extension of the strata 

 affected. It is, however, in the sequence, rather than in the 

 character, of the faulting that we must seek for an explanation 

 of the movements that broug-ht about the present structure of 

 the part of the Don Valley now under investig-ation. The 

 Southerly Don Fault ends abruptl}' against the Sheffield one, so 

 the last mentioned is evidently the older. The Southerly Don 

 Fault is perhaps the niost interesting- in the coal-field, and there 

 is a strong suspicion * that it was progressive in its character, 

 and moved more than once between the deposition of the 

 Barnsley coal and the laying down ot the Permian rocks. That 

 is, however, a question that does not concern us here. The 

 important point is that its extension into the area under dis- 

 cussion was posterior to the Sheffield Fault and anterior to the 

 Northerly Don Fault. 



When the fracture was extended, after the close of the middle 

 coal measure period, to use the conventional term, up to the 

 Sheffield Fault, it did not establish complete equilibrium in the 

 stressed rocks, for afterwards the strip, that now lies with a 

 strike at right angles to the rest of the district, broke away. 



* Kciuliill, '(J.J.(;.S.' .\o. J4J, May 11)05, P- .^44- 



Naturalist, 



