Our Coalfields: Present and Future Prospects. 5517 
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 Sheffield district 
published this year,? Professor Fearnsides directs attention to 
a research by Sorby, 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~ slime, as Sorby’s quantitative experiments 
seem to indicate, is it not possible that the discordance we are 
discussing between the Middle and Upper Coal-measures is due, in 
part at all events, to differential contraction and consequent local 
sageing during the extremely slow squeezing out of the water by the 
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 sub- 
aqueous, producing a non-sequence similar in character (and origin 
perhaps) to the relatively small stratigraphical breaks which have 
been recognized 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 
1 The Keuper Marls around Charnwood, pp. 47-50, 1904-11. 
2 Trans. Inst. Min. Eng., vol. 1, pt. iii, 1916. 
> Q.J.G.S., vol. lxiv, pp. 171 et seq., 1908. 
