282 Mr. S . V. Wood on the Events which produced and terminated 



Skye, the direction of the line of subsidence which prevailed 

 over the same area through the cretaceous age, until the great 

 continental upheaval commenced at the epoch of the Msestricht 

 chalk, is indicated by the parallel drawn almost at right angles 

 to the former line, and extending from Dorsetshire to Valen- 

 ciennes, coinciding in direction precisely with that of the axis 

 of cretaceous elevation in the Pyrenees, to which it may be 

 regarded as the complementary line of depression. We also see 

 that this subsidence was at its maximum over the line of the old 

 palaeozoic barrier, with whose anticlinal it very nearly coincided, 

 and that it sensibly diminished as the distance from that line 

 increased ; while along the line of Jurassic elevation neither sub- 

 sidence nor elevation took place, the volcanic action of that band 

 having become extinct at the close of the oolitic period, and the 

 movements of the band theretofore affected by it having conse- 

 quently terminated. We thus perceive how the ocean, kept out of 

 the Wealden basin during the progress of that formation, did 

 not, when the subsidence of the barrier permitted it to re-enter, 

 overflow the outcrop of the upper oolites surrounding the basin — 

 an overflow which never more than partially took place, and that 

 not until the barrier had been submerged sufficiently to receive 

 the deposits of the gault and upper greensand over it. By the 

 close of the epoch of the chalk with flints this barrier had undergone 

 a depression from its condition at the close of the Purbeck epoch 

 of little less than 3000 feet. The maximum of depression varied 

 over the line itself during the earlier portion of the lower creta- 

 ceous age, since, while a depression ensued at that time in 

 Dorsetshire sufficient to deposit a great thickness of Wealden 

 formation over the Purbeck, no further depression of any mag- 

 nitude ensued at that extremity of the line until the upper 

 greensand epoch. From the concentration of the action of 

 subsidence in different parts of the barrier at different stages 

 of the cretaceous age which appears to have taken place, and 

 particularly from the chief concentration appearing to have 

 occurred in the part between London and Valenciennes, it 

 does not seem necessary to infer any very considerable elevation 

 of the bed of the north-east strait during the progress of the 

 Wealden deposit. In Oxfordshire, across the south of which 

 county I place the northern margin of the lake, and near which, in 

 the adjoining counties of Buckingham and Bedford, I place the 

 north-east opening, the lower greensand underlying the upper 

 cretaceous deposits occurs, according to Prof. J. Phillips*, in a 

 very attenuated form, showing, unless the flow of the tide pre- 

 vented the accumulation of sediment, how little depression that 

 extremity of the barrier underwent during the lower cretaceous 

 * Quart. Journ. Geol. Soc. vol. xvi. p. 30/. 



