XCVm PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



in the uppermost of a series of beds which the author considers must 

 have been part of a marginal sea-zone. This latter conclusion, how- 

 ever, appears very doubtful, as it is by no means probable that such a 

 growth of trees as is visible in this, the Portsmouth, and many other 

 cases could have belonged to a sea- zone or margin ; on the con- 

 trary, it is far more probable that these localities were then sheltered 

 spots, secured from the sea-blast by an advanced line of coast. 



It is unnecessary that I should follow Mr. Austen as he endeavours 

 to trace out the several oscillations of the surface, by which perhaps 

 a depression was produced here, and an elevation there, tending at 

 one point to change a terrestrial surface into a marine bottom, and 

 at another to place a marine bottom in the conditions of a terrestrial 

 surface ; and still further to distinguish from the effects of such 

 changes the alterations produced either simultaneously, or before or 

 after the last undulation, by the action of the waves ; but I shall briefly 

 notice some of the elements of the inquiry. I have before pointed 

 out the distinction between the gravel of crystalline rocks and that 

 of the chalk with its imbedded flints ; in some cases the pebbles of 

 the former occur in the latter, and this might reasonably have been 

 expected on the mere supposition of depression, as the highest 

 shingle woidd necessarily have been then exposed to the action of 

 the waves and removed to lower levels ; but Mr. Austen thinks it 

 impossible to refer these pebbles either to the western rocks of Eng- 

 land or to those of France, and states his belief that the ancient 

 coast-line along which these pebbles had drifted was neither that 

 of the west of England nor that of France, but was in fact con- 

 nected with a mass of old rocks which constituted an eastern ex- 

 tension of the axis of the Channel area, now submerged ; and I need 

 scarcely add that a similar supposition has been advanced in expla- 

 nation of certain climatal differences observable in the tertiary de- 

 posits, which, it is presumed, might have been the consequence of 

 the sudden removal of a barrier which had before shut out the warm 

 western waters from the regions previously occupied by the waters 

 of the North Sea. That a barrier has been removed can scarcely be 

 doubted ; and that the removal of that barrier may have been assisted 

 by a line of fracture, produced either by elevation or depression, 

 is almost equally certain ; but the pebbles of this shingle seem a 

 slight basis for the assumption of the existence of a former mass 

 of old rocks in this direction. One curious illustration of depression 

 is derived from the form of the bottom of the Channel, which Mr. 

 Austen represents as exhibiting " Hues of troughs and an advancing 

 platform indicative of an old coast- line ;" but it is doubtful whether 

 mud- deposits on a coast would retain on submergence this marked 

 character, and it is equally probable that submarine currents would 

 tend to produce and preserve a cliff-like character in the lower 

 deposits. A more striking proof is derived from the mollusca 

 in the Selsea deposits, which indicate, as Mr. Austen observes, 

 by their habits, shallow water and marginal conditions, that the 

 eastern extension of the Channel at that period may be represented 

 by a line extending from the coast of Sussex to that of Normandy, 



