98 



layer of disturbance I mentioned, and I have no hesita- 

 tion in assigning it to an earlier fit of glaciation than that 

 one. I have drawn attention some time ago to a faulted 

 and overturned mass of pebble beds at the top of the 

 southernmost face of the Great Ballast Pit near Erith 

 Station, and I think that it may be referred to one of the 

 earlier glaciations {PI. \\. fig. i). It lies between two slight 

 valleys of unequal depth, and at a point which, before 

 the present excavation extended so far, constituted their 

 place of junction. The mass of eocene sands and pebbles 

 has been pushed from some point a few yards southward, a 

 depression behind the mass now filled with at least 12 

 feet of rainwash or loam, shews that a large mass of ice 

 must have cut rather deeply into the earth. I do not 

 think that this was done by. shore ice. 



Having now shown recurrence in land glaciation as I 

 take it, and given some evidence as to its great destructive 

 power, I am in a position to consider the present condition 

 of the dry valleys of the chalk. 



It has been asserted and generally accepted that the 

 dry valleys of the chalk have been hollowed by subaerial 

 agencies, similar to those which produced the clay with 

 flints, I think this an insufficient explanation ; doubtless 

 they have been the seat of subterranean drainage deter- 

 mined by joints and lines of weakness (see ante) but if 

 the dissolution of the chalk by chemical agency had any 

 considerable share in their excavation we ought to find in. 

 their bottoms, clay-with-flints ; and whole unfractured 

 flints, like those at the base of that clay on the upland, in a 

 very much greater quantity, proportional to the greater 

 quantity of the chalk dissolved; but it is not so, no clay 

 with flints enveloping perfect blocks of flint is there. 



It is certain that these valleys for an immense period 

 of time have not been open water-courses any more than 

 at present, nor could water course on the surface of the 

 chalk. It remains then to account for much of their 

 formation by another method. After the evidence I have 



