136
clays resemble that which might be brought down from decomposinggranitic or basaltic districts, probably from some land to the S.W.,towards Brittany or the coast of Spain . The general aspect of alarge proportion of the mottled dark red clays favours this view,which is further corroborated by the fact before alluded to of thepresence of gelatinous silica, or silica in that condition in which we maypresume it to be when derived directly from the decomposition offelspathic rocks, without having gone through any intermediategeological stage. The influence of such river-action was evidentlygreater in Hampshire than in Berkshire, the beds in the formercounty being of nearly double the thickness of those in the latter, andfar more homogeneous. Compared to the Woolwich clays, the massof materials forming the mottled clays is out of all proportion larger.Its arrangement is also very peculiar, its lines of bedding beingalmost always waved and curved, as though brought down anddeposited by fits and starts, as by the freshets of a large river. Thesmaller mass of sands with which they are interstratified were pro-bably brought down continuously by the Weald-Island rivers, chieflyfrom an area of Lower Greensand, spread by sea currents, and thusintercalated with the great mass of these mottled clays derived fromthis other more distant source. The almost total absence of car-bonate of lime and the presence of the gelatinous silica are causesprobably sufficient to account for the absence of organic remainswherever 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 strataforming the Basement-bed of the London clay over the whole of thevaried 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 ofthe like 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 duringthe period of the London Clay, apply in some measure, but proba-bly less forcibly, to this intermediate epoch of the “ Woolwich andReading series.” The general character both of the fauna and florashows a preponderance of forms such as, on the whole, we mightexpect to meet with at present in more moderate climates than theone in which the more tropical-seeming vegetation and animals ofthe London Clay could have flourished. For a subject, however,of this problematical nature, the data are too limited to arrive at anyvery satisfactory or definite result. I merely state the general im-pression, rather than any sufficient conviction, I have received fromthe inquiry into this subject.
* Quart. Journ. Geol. Soc. vol. viii. p. 260.