MACKIE — THOUGHTS ON DOVER CLIFFS. 
287 
their food, and there oft during the sestival droughts millions of 
marsh-mollusks 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 Tguanodon, 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. 
Purther 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. 
