392 LAMPLUGH : EARTH-MOVEMENT IN NORTH-EAST YORKSHIRE. 
greatest in the Yorkshire area where the Secondary strata were thickest; 
and it is the same in the Weald and in the South Midlands of England. 
We may call in the weight of the sediments to explain the progressive 
deepening of the basin, but the idea fails us, here as elsewhere, when 
we come to deal with the whole history of the area. If, under certain 
conditions, extra weight be operative in the production of such basins, 
its operation is clearly not cumulative, and can be overpowered and 
obliterated by other factors, whatever these may be.* In applying 
this " theory of isostasy " (as it is sometimes called), there is obviously 
a danger of confusing cause and efiect. Sediments would tend to 
accumulate within a basin lying in their path, whatever its origin ; 
and they would continue to gather there so long as it continued to 
deepen. How are we to distinguish between a case in which the 
position of the sediments is due to the presence of the basin, and one 
in which the position of the basin is due to the presence of the 
sediments ? 
Reverting to the diagram Fig. 1, it is obvious that when the Upper 
Cretaceous plane was horizontal, all the underlying Jurassics in the 
line of section must have had a northerly dip, if we may rule out the 
uncertain factor of concealed pre-Cretaceous faulting. Therefore the 
present general southerly dip of the Lias and Oolites between the 
valley of the Esk and the Vale of Pickering, if measured from the 
horizontal, does not give the full value of the relative northern uplift, 
but requires to be supplemented by the amount of the previous 
opposite dip. The northern upheaval appears to have pivotted on 
nearly the same hinge-line as the preceding subsidence, and regained 
in this quarter more than was lost. But it has not wholly obhterated 
the original basin. The bold westerly bulge of the Jurassic outcrops 
between the Tees and the Humber is to some extent indicative of the 
position of the deeper part of the basin ; and this is still more clearly 
displayed if we plot the present profile of the Rhsetic plane along a 
line drawn south-south-eastward from the mouth of the Tees to the 
western corner of the Chalk escarpment, as I have done in Fig. 2 (PI. 
XLIV.). This plane has, of course, been affected by every movement 
* Some of these difficulties in the theory of -isostasy have been 
recently discussed from the physicists' standpoint by Dr. H. Jeffreys, 
in a speculative paper " On Certain Geological Effects of the Cooling of 
the Earth," Proc. Boy. Soc. Series A, Vol. 100, No. A703 (1921), 
pp. 122-149. 
