Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/102

 and the Nore ought to form but a subordinate proportion of the constituent material, while in the Canterbury-heights gravel nearly as much subcretaceous material as flint ought to occur. The case of the East-Essex gravel, especially of that part of it which extends from the Nore to Rochester, is a very strong one ; for the chalk forms but a very small part of the area drained by the Medway, and, while the gravel-producing material, the flint, constitutes only a small percentage of any given amount of chalk strata removed, especially of the Lower Chalk which obtains in this area, the portion of the Lower Greensand formation which is drained by the Medway is largely made up of beds of hard stone. The Hastings-Sand formation, too, abounds with indestructible gravel-forming material, and in as large a ratio at least as does the chalk. Omitting the Weald Clay and the Gault as non-gravel-forming strata, we have, roughly speaking, the following proportions borne by the areas of those gravel-producing formations lying beyond the East-Essex gravel termination at Rochester whose drainage falls into the Medway, viz. : —

Lower London Tertiaries 0.25

Chalk 1.00

Lower Greensand 2.75

Hastings Sand 3.00

Total 7.00

While the chalk thus figures for only one-seventh in area, it would, in proportion to any given quantity of strata removed, yield no more, indeed less, of flint than the Lower-Greensand beds, or even the Hastings-Sand formation, would of hard gravel-forming material.

If it be objected that the stone beds of the Lower Greensand are mostly limestone, and therefore soluble under the action of acidulated water, such objection does not apply to the Hastings-Sand material, of which, indeed, the broad sheet spreading over the Weald-Clay bottom is mainly, and in some parts exclusively, composed. Neither has it prevented the gravels of the Lower Greensand country from being principally made up of the stone beds of this formation. Moreover, the Kentish Lower-Greensand Limestone, so extensively used in building, is not of a perishable nature, and much of it is in that broken condition most suitable for supplying fragments for gravel-accumulation; while on the other hand so perishable a material as the Kentish Chalk has, according to Messrs. Topley and Foster, found its way, in the form of nodules, into gravels near Maidstone. Allowing, therefore, the fullest weight to this objection, can we resist the admission that if the East-Essex gravel, especially that part of it lying between Rochester and the Nore, resulted from the transport of the Medway, the flint debris in it (exclusive of the Lower Tertiary pebbles) should be largely outbalanced by subcretaceous material, instead, as the case is, of that material forming but a very small proportion of this gravel ? This inference will not be appreciably weakened by supposing that the respective escarpments extended southwards in former time, because a careful examination of