136 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



clays resemble that whicli might be brought down from decomjx»sing 

 granitic or basaltic districts, probably from some land to the S.W., 

 towards Brittany or the coast of Spain. The general aspect of a 

 large proportion of the mottled dark red clays favours this view, 

 which is further corroborated by the fact before alluded to of the 

 presence of gelatinous sihca, or silica in that condition in which we may 

 presume it to be when derived directly from the decomposition of 

 felspathic rocks, without having gone through any intermediate 

 geological stage. The influence of such river-action was evidently 

 greater in Hampshire than in Berkshire, the beds in the former 

 county being of nearly double the thickness of those in the latter, and 

 far more homogeneous. Compared to the Woolwich clays, the mass 

 of materials forming the mottled clays is out of all proportion larger. 

 Its arrangement is also very peculiar, its lines of bedding being 

 almost always waved and curved, as though brought down and 

 deposited by fits and starts, as by the freshets of a large river. The 

 smaller mass of sands with which they are interstratified were pro- 

 bably brought down continuously by the Weald-Island rivers, chiefly 

 from an area of Lower Greensand, spread by sea currents, and thus 

 intercalated with the great mass of these mottled clays derived from 

 this other more distant source. The almost total absence of car- 

 bonate of lime and the presence of the gelatinous siUca are causes 

 probably sufficient to account for the absence of organic remains 

 wherever these mottled clays prevail. 



After a time this Lower Tertiary period was brought to a close, 

 and its islands, with their streams, whose action we have been studying, 

 were submerged, by the great movement of subsidence, at first rapid, — 

 and productive of the transport of the conglomerate and mixed strata 

 forming the Basement-bed of the London clay over the whole of the 

 varied surface of the Woolwich and Reading series, — and which sub- 

 sidence was afterwards continued by that quiet and prolonged move- 

 ment, which we have shown to be necessary for the accumulation of 

 the hke materials of, and transmission of a like fauna throughout, 

 the great mass of the overlying London clay. 



The reasons for believing that the temperature of the sea at the 

 " Thanet Sands" period* was lower than that which prevailed duruig 

 the period of the London Clay, apply in some measure, but proba- 

 bly less forcibly, to this intermediate epoch of the " Woolwich and 

 Reading series." The general character both of the fauna and flora 

 shows a preponderance of forms such as, on the whole, we might 

 expect to meet with at present in more moderate climates than the 

 one in which the more tropical-seeming vegetation and animals of 

 the London Clay could have flourished. For a subject, however, 

 of this problematical nature, the data are too limited to arrive at any 

 very satisfactory or definite result. I merely state the general im- 

 pression, rather than any sufficient conviction, I have received from 

 the inquiry into this subject. 



* Quart. Jouru. Geol. Soc. vol. viii. p. 260. 



