STUDIES FOR STUDENTS 179 



vast mud fiats upon which the St. Regis beds were being laid down, 

 the surface was raised up into a low dome of small extent upon which 

 the soft strata were exposed to wave action and fragments were 

 broken away and incorporated with the soft siliceous mud that 

 was then accumulating in the surrounding waters." 



Reef breccia. — -Reefs with brecciform structures may be built 

 by corals, calcareous algae, and molluscous shells. Coral breccia, 

 the variety most common and most closely studied, is produced in 

 several different ways. As a coral reef is built up toward low- tide 

 level, the interspaces between the masses of growing coral are filled 

 with broken fragments of coral branches and the finer waste of the 

 reef. The coral framework is brittle and is further weakened by 

 boring worms and mollusks ; hence the accretion of broken branches 

 goes on below the zone of wave-wear, and the fragments remain 

 angular. After wave-base is reached, accretion proceeds still more 

 rapidly, and now the other rim of the reef, the belt of its most active 

 growth, acts as breakwater and protects the inner portions of the 

 coral fields from the wear of the surf. Thus is formed reef -rock 

 breccia or coralline rag, a well-cemented limestone in which masses 

 of coral retain the attitude and position of growth, and to which 

 the varied animal and vegetal life of the reef contributes. 



In this reef-rock waves cut the channeled and cavernous rock- 

 bench. The fragments plucked from the bench are swept inland 

 by heavy storms over and beyond the beach of coral sand, and 

 cover large tracts with lichen-blackened fragments, angular to 

 such a degree that both Dana 1 and Sollas 2 have compared them 

 to the rough clinkers of lava which strew the slopes of Mauna Loa 

 and of Etna. Intermixed is wave-worn and wind-blown coral 

 sand, which acts as a matrix, cementing the breccia of the island 

 rock. By slow subsidence these deposits may be carried beneath 

 the surface of the sea. The upgrowth of the rim of the reef mean- 

 while protects them from being worked over by the waves and thus 

 the brecciated stucture is preserved. 



A third variety of coral breccia accumulates at the foot of the 

 steep outer face of the reef, where angular fragments torn by waves 



1 J. P. Dana, Corals and Coral Islands (New York, 1879), p. 178. 



2 W. J. Sollas, Age of the Earth (London, 1905), chapter on Funafuti, p. 108. 



