MACKIE — THOUGHTS ON DOVER CLIFFS. 



287 



their food, and there oft during the sestival droughts millions of 

 marsh-tnollusks on the half-dried banks retreating into their spiral 

 homes were baked by the hot sun into beds of beautiful marble (Be- 

 thersden and Purbeck), so often since polished into pillars for holy 

 places, or chiselled into seats for saints. 



In the lapse of further ages — for Time's march has never stayed — 

 the salt waves curled and broke in writhing foam, and the sea-breezes 

 rippled the ebbing tide over the shallow sands, which the rain spat- 

 tered and the creeping things of the shore streaked and dotted with 

 their footprints. 



Down further yet went the ancient land, and the mud took the 

 place of the sand ; further still and the fine calcareous ooze of a 

 deeper ocean succeeded to the mud ; and the river-mussel and the 

 reptile were supplanted by the ammonite and the shark ; the trees 

 and insects by the mollusk and foraminifer. 



The ancient trees of that old continental land are still dug up in 

 the Purbeck beds ; the bones and teeth of the Iguanodon, the Ptero- 

 dactyle, and other gigantic reptilia are often exhumed from the sands 

 and clays of the Wealds of Kent, Surrey, and Sussex ; and the rip- 

 ple-marked sandstone is daily quarried at Horsham, Hastings, and 

 Eye. Ammonite and shark, crab and shell-fish have left their 

 solid cases, bones, and teeth in the ragstone, greensand, and gault, 

 and even traces of the soft flesh of the latter yet remain as brown 

 nodules of molluskite in some of the sandy beds of stone. 



Further still sank the ancient land with its overpiled sediments 

 until the faint currents of the ocean-deeps scarcely stirred the tiny 

 valves and weightless shells of microscopic entomostracs and fora- 

 minifers,* powdering the fine calcareous mud in depths which the 

 light of heaven only dimly reached. 



All these various beds, formed during the vast period of those 

 strange changes, however dissimilar in their present state they may 

 at the first glance appear, both in mineral composition and in the 

 physical conditions of their deposit, are nevertheless found to be 

 naturally associated, when the sequence of their succession and the 

 nature and value of their organic remains are known. 



No general distinguishing mineral characters can be universally 

 assigned to these various divisions, for the composition of the beds of 



* Some portions only sank to this great depth. The area of Kent and Sussex was 

 nearer the old continent, and consequently sank into less profound though still deep 

 water. 



