﻿1851.] PRESTWICH ON THE DRIFT AT SANGATTE CLIFF. 277 



and Tertiaries, might, if the distance were not great, transport, com- 

 paratively uninjured and unbroken, masses of the softer strata, and 

 even the more delicate shells which they contain. The condition of 

 the mass would depend, therefore, entirely upon the distance upon 

 which it had travelled, and the nature of the ground over which the 

 waters passed. 



If we suppose a large body of water to be moving with a velocity 

 sufficient to transport large blocks, then, necessarily, the smaller 

 debris and the mud and silt would be carried along with them ; but 

 when the velocity is only sufficient to move the finer debris, then the 

 coarser materials and larger blocks must be left behind. In the 

 former case the heavier portions would subside first, and the lighter 

 ones be carried to a greater distance and become gradually more worn. 

 But let this current be arrested in the early part of its course, and 

 then we shall have a deposit of mingled debris, the less sorted the 

 nearer we approach to its point of origin ; and if this should have 

 been effected after a short transport, and without meeting with any 

 material impediments, then masses of clay and sand, with their im- 

 bedded organic remains, however delicate, as well as the bones of 

 animals occurring on the surface of the ground, may, I apprehend, 

 be transported comparatively uninjured and unbroken. Should how- 

 ever any impediments occur to obstruct their progress, or any con- 

 flicting currents disturb the uniform and regular sweep of the moving 

 mass, then the clash of the debris will more or less break and wear 

 both the organic remains and the rock-detritus in proportion to their 

 hardness and power of resistance ; the more friable and delicate speci- 

 mens being first destroyed, and only the harder bones and rock-debris 

 holding out to the last. The whole mass would also become more in- 

 termingled. 



In the case of the Sangatte Drift, the materials do not appear to 

 have travelled far, the lower portion of it consisting chiefly of pure 

 chalk-rubble, derived from the adjacent Lower Chalk, and the upper 

 part being full of flints which probably came from a rather more 

 distant point of the same range of hills. The whole mass seems 

 to have been checked and thrown down after but very little wear, and 

 in a manner comparatively independent of the specific gravity of its 

 component parts ; whereas, if the action which accumulated these 

 materials had been slow, gradual, and long-continued, they would 

 most likely have been sorted according to their specific gravities ; and 

 more particularly as all the fragments in any given layer would have 

 been subjected to a force acting comparatively with equal power, and 

 in an equal manner, they would necessarily all exhibit a nearly like 

 amount of wear, varying in the different specimens according to their 

 hardness, but the measure would be alike for all. In this and the 

 analogous case at Brighton, an impalpable chalk-silt would, I appre- 

 hend, be incompatible with coarse siliceous sands — worn pebbles with 

 sharp angular flints — the largest debris with the finest — and entire 

 and perfect bones with others, some of which are broken and others 

 rolled and worn. 



At the same time the bed of chalk-flints which reposes imme- 



