334: DR. A. M. DAVIES AND ME. J. PRLNGLE ON [June I913,. 



V. The Palaeozoic Floor north of the Thames. 

 [A. M. D.] 



The time has come when it may be useful to take stock of the- 

 knowledge already gained from scattered borings as to the depth 

 of the Palaeozoic floor, and to express known facts and probabilities 

 in the form of a contoured map. Where Triassic deposits rest upon 

 it, this floor probably has the uneven form of a buried land-surface,, 

 much too irregular to be contoured from scattered observations ;. 

 but, where marine strata directly overlie it, it is probably a smooth 

 peneplain — either of prolonged subaerial or of marine denudation. 

 Further, where Trias is present, its deposition must have levelled up 

 the irregular land-surface to another smooth plain. In short, the- 

 base of the marine Mesozoic (of whatever age, from Bhaetic to Gault) 

 must form a sufficiently continuous smooth surface to be contoured 

 from its known outcrop to the depths at which it is met in borings. 

 This is what has been attempted in PL XXXIV. Where Trias is 

 absent, the contours are those of the Palaeozoic floor. 



On the same map have been indicated, as far as the scale would 

 allow, all the mapped folds and faults in the Mesozoic rocks, andi 

 the chief of those in the exposed Palaeozoic rocks. These are of 

 importance for a consideration of the possible tectonic structure of 

 the buried Pakeozoic floor, if that of its Mesozoic cover corresponds- 

 to it in any degree. 



The danger of assuming too simple a structure in the buried 

 Palaeozoic floor is evident, if we consider the repeated series of 

 earth-movements to which its constituent rocks may have been 

 subject : — 



(1) Post-Tremadoc movements, indicated by the absence of Ordovician, 



rocks south-east of Shropshire ; 



(2) Post-Lower Devonian movements, indicated by the barrier between 



the Midland and South- Western Provinces, and the unconformable- 

 sequence of Coal Measures on Lower Old Eed Sandstone in South 

 Staffordshire and elsewhere 1 ; 



(3) Eepeated intra-Carboniferous movements shown by the zonal work of 



L»r. Vaughan, Dr. Sibly, and Mr. Dixon, by the overstep of Lower- 

 Carboniferous by Upper Coal Measures in the Forest of Dean, 2 and', 

 by local irregularities in the Midlands 3 ; 



(4) Post-Carboniferous movements. 



The most striking example of continuity of tectonic lines from- 

 the Palaeozoic area into the Mesozoic area is afforded by the 

 western boundary-fault of the Warwickshire Coalfield, which has a. 

 north-and-south or Malvernian trend. This is carried on through 

 the Trias of the Forest of Arden in the form of the easternmost 



1 W. W. King & W. J. Lewis, Geol. Mag. dec. 5, vol. ix (1912) pp. 487, 488, 



2 T. F. Sibly, ibid. pp. 417 ct segg. 



3 ' The rocky floor upon which the Carboniferous rocks of the Midlands were- 

 deposited seems to have been greatly affected by the action of local crust-creep 

 during Carboniferous time ' : C. Lapworth & W. W. Watts, ' Geology of the? 

 Birmingham District' Proc. Geol. Assoc, vol. xv (1898-99) p. 364. 



