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Show 288 <" ' ~ ~ ~ i:>o. ~"' ~ !:; t:i i ~" ~ ~., ~ .a ~ .g !e 0 14 :"s' 'lj'~ ..0 ~ ~ d Q) Q.l .b..e Q) 11: 0 ~ M~ .... :3 <:! ~ c.f Q) ~ ~ \!:! ~ "d r:: Cll ,.!< Oj ,.c:: u ....:- .J ~ ~ -~ 1i ~ cf EOCENE PERIOD. [Ch. XXI. Ch, XXI.) CHALK ESCARPMENTS ONCE StU -CLIFFS. 289 Tn attempting to account for the manner in which the five secondary groups above mentioned may have been brought into thcit· present position, the following hypothesis has been very generally adopted. Suppose the five formations to 1ie in horizontal stratification at the bottom of the sea; then let a movement from below press them upwards into the form of a flattened dome, and let the crown of this dome be afterwards cut off, so that the incision should penetrate to the lowest of the five groups. The different beds would then be exposed on the surface in the manner exhibited in the map, plate 5 *. It will appear from formet· parts of this work, that the amount of elevation here supposed to have taken place is not greater than we can prove to have occurred in other regions within geological periods of no great duration. On the other hand, the quantity of denudation or removal by water of vast masses which are assumed to have once reached continuously from the North to the South Downs is so enormous, that the reader may at first be startled by the boldness of the hypothesis. But he will find the difficulty to vanish when once sufficient time is allowed for the gradual and successive rise of the strata, during which the waves and currents of the ocean might slowly accomplish an operation, which no sudden diluvial rush of watet·s could possibly have effected. Escarpments of tha chalk once sea-cliffs.-In order to make the reader acquainted with the physical structure of the Valley of the Weald, we shaH suppose him first to travel southwards from the London basin. On leaving the tertiary strata he will fit·st ascend a gently-inclined plane, composed of the upper flinty portion of the chalk, and then find himself on the summit of a declivity consisting, fot· the most part, of different members of the chalk formation, below which the upper green-sand, and sometimes also the gault c·rop outt. 'I' his steep declivity is called by geologists ' the escarpment of the chalk,' which overhangs a • See illustrations of this theory. by Dr. Fitton, Geol. Sketch of Hastings. f We use this term, borrowed from our miners, to express the coming UJI to the surface of one stratum from heucuth another, VoL, III, lJ |