122 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



mottled clays still preponderating, until, on the confines of Surrey and 

 Kent, Middlesex, and Essex, the latter are rather abruptly displaced by 

 the sands, which finally in East Kent prevail exclusively. The two 

 principal subordinate features are, 1st, the occurrence of rolled flint- 

 pebbles, black, and worn perfectly smooth and rounded *, usually of a 

 small size, -^ to 2 inches in longest diameter, but occasionally attain- 

 ing a length of 10 to 15 inches. These pebbles are generally di- 

 spersed in layers or imbedded in the sand, but they are in few cases 

 associated with the clay beds. It is within the area wherein thje 

 change from mottled clays to sands takes place that the pebbles are 

 developed in their greatest profusion, forming at those places thick 

 and extensive beds of shingle. To the eastward beyond this area 

 the sands are still characterized by dispersed pebbles, whilst to the 

 westward of it the pebbles are restricted to a thin bed, either just 

 overlying the green-coated flints which repose on the chalk, or else 

 intermingling with these flints. 2nd, Green sand, which is usually 

 either disseminated in dispersed grains, or else occurs in small patches 

 or seams, and is chiefly confined to the lower portions of this divi- 

 sion, and at the base of which it forms, mixed with clay and per- 

 oxide of iron, a thin and nearly constant layer. In Kent this green 

 sand exists in larger proportion, often constituting irregular beds 

 of light green sands, which are never, however, persistent over any 

 wide range, and are always subordinate to the light quartzose sands. 

 These grains of green sand vary in size from a pin's point to a pin's 

 head, the larger ones being botryoidal-shaped, of a malachite-green 

 colour, and rather soft. These green grains have usually been indis- 

 criminately referred to Chlorite and Green Earth, but the physical 

 characters of these grains differ so widely, — some being light green, 

 soft, opake, and earthy, whilst others are very hard, of a dark green 

 or nearly black colour, translucid, and show facets, — that I would 

 suggest whether or not, in addition to the former of these minerals, 

 there may not be, mdxed with the common quartzose sand, debris of 

 Hornblende and Pyroxenes f in this state of small grains. 



As minerals of more local occurrence may be noticed, 1st, The 

 peroxide of iron, occasionally giving an ochreous and ferruginous 

 colovir to the strata, and sometimes cementing the sands into thin 

 tabular iron-sandstones. 2nd, The hydrated peroxide of iron, or 

 limonite, in concretions, and occasionally forming ochres. 3rd, 

 Iron pyrites, which is of comparatively rare occurrence except in the 

 mixed green sands of Heme Bay. 4th, Carbonate of lime, which in 

 this group must be considered as quite a subordinate mineral, being 

 almost altogether restricted to the central area beneath and around 

 London, occasionally acting as a cement to some of the sandstone 

 conglomerates, and sometimes forming an extremel}' hard fine-grained 

 concretionary rock. 5th, Carbonaceous clays, and thin bands or rather 



* Some of these pebbles are shivered and broken in situ (usually by pressure). 

 Others again have been broken before being rolled into their present position, 

 presenting a fiactured surface with the sharp edges worn off. 



t 1 liave not had time to proceed further in this inquiry, which is one of con- 

 siderable interest in a theoretical point of view. 



