Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/352

 between them should be drawn. Mr. Godwin-Austen has remarked on the close connexion between the Upper Greensand and the Gault, and is inclined to consider their differences due rather to the conditions than to the time of deposition. Probably we should be near the truth in representing the Upper Greensand and Gault as one formation, with arenaceous characters prevailing in its lower part, and inconstant and irregularly developed beds of clay in its midst. In Kent these beds of clay are thick, well defined, and sharply separated from the sands above and below ; but in the Isle of Wight they are much less strikingly characterized ; and as we proceed eastward they thin out and finally disappear in the neighbourhood of White Nore, in Dorsetshire. Thus in the Blackdown beds of Devonshire we have the representatives of the sands above the Gault Clay and those below it brought together, a fact which may account for the anomalous character of the fauna of those beds. It is not, however, improbable that these western portions of the Cretaceous series, like certain similar deposits in the north-west of France, may be of somewhat earlier date than those of the east of England, though continuous with them, and may thus represent the latter part of that great period which must have elapsed between the Neocomian and the Cretaceous. It must be remembered, however, that the Ammonites of the Blackdown beds are nearly all well- known Gault species. My friend Dr. Lycett informs me, too, that the supposed correspondence betwen the Trigonioe of the Blackdown beds and those of the Neocomian breaks down on a careful and critical examination of the specimens. It is not improbable that some other of the supposed anomalies of the fauna of the Blackdown beds would also disappear on a more careful examination of the fossils.

3. Thinning-out westward of the Neocomian and Wealden. — Nothing can be more striking than the great difference between the thickness of the beds which lie between the Chalk and the Portland in Swanage Bay, and in the section of Man-of- War's Cove and Durdle Cove. This effect is, as we have already seen, partly due to overlap ; but in a much greater degree it is owing to the tendency which all these beds have to thin away towards the west. From the very detailed sections published by the Geological Survey * (the work of Messrs. Bristow, Whitaker, and the Rev. O. Fisher), and from my measurements, the accompanying Table has been constructed, which shows in what a very marked manner this attenuation of all the beds towards the west takes place. Of this general tendency the Punfield formation partakes. Mr. Whitaker has recently shown that the lower beds of the Chalk exhibit a similar thinning-out towards the west.

4. Relation of the Purbeck to the Wealden. — At the only points where the junction of the Purbeck and Wealden is clearly seen, namely in Worborrow Bay and Mewps Bay, the marls with Paludinoe, which form the top of the Purbeck, are seen to pass, by

and Vertical Sections, Sheet 22.
 * Geological Survey in England and Wales, Horizontal Sections, Sheet 56,