352 CORALS AND CORAL ISLANDS. 



to the presence of such an ocean continent, a set of freight 

 carriers that could beat off the waves from their accustomed 

 work, and push aside the ordinary oceanic currents ; or else 

 Atlantis would get back all its own dirt. 



VI. ABSENCE OF FOSSILS FROM LIMESTONE STRATA. 



Absence of fossils has been mentioned as a frequent char- 

 acteristic of the line compact coral reef-rock, and also of the 

 beach and drift sand-rock or oolite (pp. 153, 194). The rocks 

 are formed at the sea-level, and in the midst of abundant life, 

 and yet trituration by the action of the waves and winds has 

 in many places reduced all to the finest material, sp that an em- 

 bedded shell is seldom to be found in the beach or drift oolite, 

 and rarely too in much of the fine-grained coral reef-rock. 



The lagoon basin appears to be eminently the place for 

 making these non-fossiliferous limestones. • This is the case in 

 two widely different conditions : first, over the portions that are 

 below the coral- growing depths, which are sometimes of great 

 area ; and second, in lagoons that have become so small and 

 shallow that corals and large shells have all disappeared, and 

 the trituration is of the finest kind, producing calcareous mud ; 

 such lagoons being properly in a marsh condition. These last 

 appear to illustrate on a small scale the conditions under which 

 many of the ancient non-fossiliferous, or sparingly fossiliferous, 

 limestones were formed. 



VII. THE WIDE RANGE OF THE OLDER LIMESTONES NOT EXEMPLIFIED 

 AMONG MODERN CORAL-REEF FORMATIONS. 



Coral-reefs, though they may stretch along a coast for scores 

 of miles, are seldom a single mile in width at the surface ; and if 

 elevated above the sea, they would stand as broad ramparts 

 separated by passages mostly 20 to 200 feet deep, and often of 



