392 PRINCIPLES OF STRATIGRAPHY 



by evaporation that coral conglomerates and limestones occur. 

 Corals as such are, however, by no means restricted to the tropics. 

 Astrangea dance grows on the southern shore of Massachusetts at 

 Woods Holl, and Lophohclia prolifcra and Dendrophyllia ramea 

 form dense beds at a depth of from loo to 200 fathoms off the 

 coasts of Norway, Scotland and Portugal. 



Other Physical Conditions. For the successful growth of reef 

 corals it is further necessary that the water should be generally in a 

 state of agitation, the stagnation of the inner marginal waters of the 

 reefs or the lagoons being for the most part unfavorable to the 

 growth of coral. This is chiefly due to the fact that still water per- 

 mits fine sediment to be deposited, and this quickly kills the polyps. 

 On the other hand, very powerful surf is equally deleterious to the 

 coral polyps, for it will throw the shingles against the soft polyps 

 and so destroy them. Moreover, the heavy breakers will tear off 

 the branching madrepores, and loosen even the massive msean- 

 drinas, owing to the fact that the dead base of the colony is often 

 so much riddled by boring organisms that it easily succumbs to the 

 onslaught of the waves. When such a great mass of coral is 

 loosened, it will be rolled along the reef, rounding and grinding the 

 other corals in its course. (Bourne-14: ^^0.) 



A strong current likewise seems to be unfavorable to the growth 

 of corals, if it sets directly against them. Thus at Diego Garcia, 

 and especially at the east end of East Islet in the Chagos group, 

 strong direct currents strike the reef, which is barren here and suf- 

 fering rapid erosion, where the whole force of the current falls 

 directly upon them. "As the current parts and flows round the 

 obstacle one meets with a reef covered with debris, but barren of 

 live coral ; further on, as the current moderates in force, one finds 

 a few growing heads of coral ; and finally, at the further end of the 

 reef, where the current has abated its force considerably, there is a 

 luxuriant bed of living corals and Alcyonaria." (Bourne-14.) The 

 same facts have been observed at Celebes, and the conclusion seems 

 to be supported by many observations "that direct and strong cur- 

 rents are unfavorable to coral growth, that moderate tangential 

 currents are extremely favorable, and sluggish or still water again 

 unfavorable." A hard rock bottom is furthermore a necessity for 

 good coral growth, these organisms finding it impossible to become 

 attached to muddy or silty bottoms. 



A normal composition of sea water is required, and the normal 

 salinity of 35 permille seems to be most favorable, as in the open 

 oceans, but a surface salinity decreased to 32.1 permille, as in the 

 China Sea (volume salinity 34 permille), or even 31.5 permille, as 



