MANUAL OF NATURAL HISTORY. 589 



North-Sea, from accumulations of sea- weed. Shell- 

 beds are formed of dead and drifted shells, heaped 

 together by tides and currents ; the fresh-water 

 species producing beds of marl, and the marine form- 

 ing layers of oysters, mussels, and cockles, which are 

 often silted up and form elevated beds. Landslips 

 are often recent formations, which are produced by 

 a soft stratum being undermined by water, and the 

 harder superjacent earth sliding down, as in the 

 Undercliff of the Isle of Wight. Some recent de- 

 posits are almost entirely composed of polygastric 

 animalcules, as polishing-slate, or tripoli. The 

 Polypi/era also form superficial incrustations on the 

 inequalities of the sea-bottom, caused by lofty moun- 

 tains, or banks near the surface ; the labours of these 

 small animals forming reefs and atolls, alter consi- 

 derably the features of groups of islands. The cir- 

 cular reefs, or atolls, are founded on the summits of 

 submerged volcanoes ; the edges of the crater form- 

 ing a foundation for the reef, and the crater itself 

 constituting the central lagoon, as in the coral islands 

 of the Dangerous Archipelago. Flat or tabular reefs 

 are founded upon an elevated portion of sea-bottom, 

 and form solid islands of coral. Long and narrow 

 reefs are founded on submerged mountain-ridges, 

 and are often of great length, as on the east coast of 

 New Holland and in the Indian Ocean. Reefs en- 

 circling high land, and separated from it by a narrow 

 channel, indicate a submergence of the islands they 

 surround, as at Tahaa and Raiatea. Great masses 

 of limestone are formed by the sea- water dissolv- 



