328 ' PEOr. J. PRESTWICH ON THE KAISED 



ward ? Even in the case cited by Mr. Drew, where the mountains 

 rise several thousand feet above the valley, the cones of dejection do 

 not extend more than 1 mile from the point of discharge, while 

 at the base of the cones the debris has a thickness of 500 feet. 

 Nor is it possible that either snow or rainfall on open ground could 

 have transported the huge blocks found in the Head. 



Por these reasons I do not think that either of those subaerial 

 agencies, although meeting some of the conditions of the case, would 

 suffice to account for the whole of the phenomena, and on the other 

 hand they must also have led to results which are not in accordance 

 with the known facts. 



3. While agreeing with much of Murchison's reasoning, there is, 

 besides the objections already named, the question whether his hypo- 

 thesis does assign an adequate cause for the origin of the Head and 

 analogous deposits. It certainly suggests a force adequate to drive 

 a drift of flints and clay a considerable distance over nearly level 

 surfaces, tumultuous enough to fracture the bones, and yet, owing 

 to its short duration and being subaqueous, productive of less wear 

 of the rubble. The cause assigned is, however, obscure and not 

 supported by the evidence. Nevertheless, Murchison's paper is 

 a valuable contribution to the subject, and touches, I believe, the 

 true keystone in assuming the Head to be the result of disturbed 

 accumulation under water and due to a widespread cause. 



4. Lyell rightly considered that the Head (as seen at Sangatte) 

 extends too far from the old cliffs to have been a mere talus, but he 

 adduces no evidence in support of fluviatile agency in the structure 

 of the Brighton rubble, nor of traces of river-deposits in any of the 

 valleys at the back of Brighton. At that time the configuration of 

 the surface was nearly the same as at present ; therefore, even had 

 the strata been impermeable, the very limited drainage-area of those 

 valleys could only have given rise to small streams. But, with a 

 substratum of Chalk, no permanent river could have existed, for 

 the water-level would have been, then as now, below the level 

 of the valleys, and the rain falling on that area must have passed 

 underground and escaped on the shore -line, as it does at present, 

 leaving the upper valleys dry, with the water-level at a depth of 

 50 to 100 feet beneath the surface. Occasionally bourns might 

 have broken out in the lower part of the valleys, but nothing more. 



There is no doubt that during a period of much cold the rain- 

 waters would run at once from off the frozen surface of the Chalk 

 hills, but that is not sufficient to prove Mr. Clement Reid's suggestion, 

 which also is based upon sections in the Chalk area alone, and does 

 not take cognizance of the general phenomena in other localities. 

 Besides being liable to some of the objections I have urged in the 

 other cases, such as the condition of the organic remains, the 

 fluvial interpretation of Lyell and Mr. C. Reid would centre the 

 'Heads ' at the mouths of valleys, whereas their chief development 

 is at the base of slopes connected with hill-ranges. 



We must therefore seek for some other explanation than these to 

 account for the origin of the Rubble-drift forming the Head and 

 the contemporary inland drifts. 



