Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/103

 a map of the drainage-areas and of the elevations will show that the proportions home by the Chalk, Lower-Greensand, and Hastings- Sand superficies would then remain pretty nearly the same as now — the chief sufferer by such extension being the Weald Clay, owing to the steep upthrow possessed by the Hastings-Sand formation.

The question then naturally arises, how did any fragments having their parentage within the Weald get into these gravels unless there was an outflow from the Weald ? The explanation offering itself is that the tidal flow up the inlets in which I regard these gravels as having been deposited would bring such material in moderate quantities from any exposures of the parent rock within the Weald ; and I may observe here how little effect geologists seem disposed to attribute, whether in the way of transport or of denudation, to this powerful and uniform force — the tide. It seems to me that the character and contents of the main mass of the gravels of the Thames, East Essex, and Canterbury heights, composed as they are almost wholly of flint in all stages of wear, from the subangular fragment down to the spherical Lower Tertiary pebble*, is far more consistent with a derivation from the wear of a long coast-line of Lower Tertiaries and Chalk than with a derivation from rivers draining, as those of the Weald do, extensive areas of subcretaceous strata abounding in stony beds.

I now propose to consider the case of the detrital beds within the portion of the Weald here under consideration.

As before mentioned, Sir Roderick Murchison has shown the great extent and quantity of angular chalk flint which is scattered over the Lower Greensand of the western extremity of the Weald, and that flint and Lower-Greensand debris, with some Tertiary pebble, is scattered over the Weald-clay zone drained by the Eden. In the north-eastern part of the Weald, although angular flints are abundant, the gravels which I regard as anterior to those resulting from the present rivers are more or less mixed with pebbles derived from the Lower Tertiaries.

The especially noteworthy feature connected with this intermixture, however, is that the pebbles and the angular flint present no intermediate grades of rolling to connect them ; so that it is obvious this admixture of angular flint and tertiary pebble cannot, in finding its way to the positions it occupies, have undergone any considerable or repeated amount of wear by transport†. This feature seems to me repugnant to any presumption that these pebbles have settled

greatly from those of the main mass, as their flints are far coarser and more angular, and present less gradation towards the Tertiary pebbles mixed with them.
 * In this respect the Lower or Fluviatile gravels of the Thames sheet differ

† Instances occur, moreover, in which chalk fragments have occurred in this intermixture. Considering how impossible it is for chalk to sustain without dissolution any long-continued aqueous action, this circumstance is also of much importance. The difficulty is enhanced in the case of one of the gravel patches shown in the map as resting on Weald clay west of Yalding, in which Messrs. Foster and Topley speak of chalk nodules having occurred. Their transport there, however, by drainage, in the reversed condition shown in the map, seems to me simple enough.