260 PHYSICAL GEOLOGY 



are separated from the shore by a channel or lagoon are 

 barrier reefs. Rudely circular reefs inclosing a central la- 

 goon are atolls. 



Coral limestone is also of several kinds. As the reefs are 

 built up toward the surface of the sea, they are eroded by the 

 waves. The larger wave-worn fragments gather near the reef, 

 and may be cemented into firm rock by the deposition of cal- 

 cium carbonate from the sea water. Fine coral mud, ground 

 up by storm waves, is deposited over wide areas at a distance 

 from the reef, and when solidified forms a dense, fine-grained 

 limestone. On coral beaches the sea water sometimes de- 

 posits concentric layers of calcium carbonate around particles 

 of sand. A rock composed of tiny spheres that have been 

 built up in this way is called oolite. Oolitic texture is pro- 

 duced also in other ways. Figure 276 shows the general 

 distribution of coral muds and sands. 



Since their appearance in an early geological period, corals 

 have been important rock makers. Ancient reefs with all the 

 characteristics of modern ones occur, for example, near Mil- 

 waukee, Wisconsin, and at Louisville, Kentucky, where they 

 occasion the rapids in the Ohio River. The geography of 

 these places at the time the corals lived may be inferred from 

 the conditions which govern the present distribution of reef- 

 building corals. 



Red clay. The most extensive ocean deposit, covering over 

 51,000,000 square miles, is known as red clay. It is character- 

 istic of the deeper sea, remote from land (Fig. 276). Red 

 clay consists in part of the insoluble residue of shells, and in 

 part of the products of the alteration and decay of volcanic 

 ash, dust, and pumice, which floats until it becomes water- 

 logged, and is drifted great distances by ocean currents. 

 Fine dust from the land and from the combustion of meteor- 

 ites during their passage through the air also contributes 

 with extreme slowness to the deposit. 



It is important to note that few, if any, of the rock systems 

 of the land correspond to the deposits that are now making 



