tRESlDENTIAL ADDRESS. 385 



line strictly justified by the facts. In the one case the curvature of the basin 

 is made too great, and, in the other, the dips in the Middle Measures are unduly 

 increased ; for, as mining plans show, the base of the Upper Measures is by no 

 means horizontal. The fact is that the undulations in the measures throughout 

 the coalfield are extremely slight, there being scarcely any perceptible dip in the 

 strata, as noted by Scott, except near what is called the ' Limestone Fault,' 

 where the dips, as will presently appear, can be otherwise accounted for. 

 Furthermore, there is a significant absence of faults other than those which affect 

 Middle and Upper Measures equally. 



I believe there is another and a simpler explanation of this classic disturb- 

 ance, and one which harmonises, in part, the views of both Scott and Clarke ; 

 and at the same time helps to give us a reasonable interpretation of the appa- 

 rently conflicting statements which have been made by working geologists 

 respecting the relationship of the Coal Measure divisions in the Midlands. 



The Keuper Marls of the Midlands occur either in horizontal or very gently 

 undulating sheets, but Dr. Bosworth has shown that around Charnwood Forest 

 they dip in all directions, 'sometimes to the extent of 20 or even 30 degrees,' 

 and that everywhere the inclination is in the direction of the rock-slope beneath, 

 though always at a smaller angle than the slope. This local dip (or ' tip,' as he 

 calls it) ' seems most likely to have been largely caused by contraction of the 

 marls under pressure and by loss of moisture.' ^^ 



In a paper dealing with the Coal Measures of the ShefiSeld district published 

 this year,'" Professor Fearnsides directs attention to a research by Horby, 

 embodied in a memorable contribution to the Geological Society of London in 

 1908 " upon the contraction of clay sediment due to loss of water. It appears 

 to me that the penetrating genius of Sorby, with that clarity of vision which 

 comes from patient and exact quantitative experiment, may help us to clear 

 up some of the difficulties to which I have referred. If the Coal Measure 

 clays have lost something like five-sixths of the original thickness they possessed 

 as mud or slune, as Sorby's quantitative experiments seem to indicate, is it not 

 possible that the discordance we are discussing between the jMiddle and Upper 

 Coal Measures is due, in part at all events, to differential contraction and 

 consequent local sagging during the extremely slow squeezing out of the water 

 by tlie pressure of overlying sediment ? We must remember that the Middle 

 Coal Measures consist essentially of clays, and that over a large part of the 

 Midlands they were deposited on a very uneven floor, and that to start with 

 they were therefore of very variable thickness. It is easy to see, also, that an 

 arenaceous fringe of sediment where the measures abut against a rise in the floor 

 would suffer far less vertical contraction from this cause than the clay, because 

 of the very diminished ' surface energy ' of the constituent sand particles, and 

 that this would have the effect of accentuating the dip due to the sag. 



It is to be noted that Scott's observations and the bulk of his section referred 

 to the central parts of the coalfield, while Clarke deals primarily with the district 

 just north of Madeley and along the south-eastern fringe of the ' Limestone 

 Fault,' which may prove to be, in my opinion, in its early stage at all events, 

 a pre-Coal Measure ridge of limestone. 



It is quite possible, indeed probable, that portions of the undulating surface 

 of the Middle Coal Measures suffered local erosion, which, however, need not 

 imply folding of the beds with prolonged subaerial denudation; for it seems 

 likely that such local erosion was subaqueous, producing a non-sequence similar 

 in character (and origin perhaps) to the relatively small stratigraphical breaks 

 which have been recognised recently in the Jurassic strata in the West of 

 England and elsewhere. 



Thus, in North Staffordshire, where the Midland Coal Basin is deepest, no 

 break between the Upper and Middle Measures exists ; but approaching the 

 southern margin of the basin, to the south of the South Staffordshire Coalfield, 

 where the Middle Coal Measures are rapidly thinning, there are, if Mr. Kay's 

 observations are correct, signs of a non-sequence or local unconformity. The 



" The Keuper Marls around Charnwood, 1904-1911, pp. 47-50. 



" Tra7xs. Inst. Min. Enq., vol. 1., Part 3, 1916. 



" Q.J.G.S. 1908, vol. Ixiv., pp. 171 et sea. 

 1916 C C 



