SUMMAKY OF REEF CHARACTERS 345 



general asi)ect they are roughly lens-shaped to dome-sliaped masses of 

 calcareous material, devoid of regular structure, without anj^, or with 

 onh^ faintly developed stratification, and composed of corals, hydro- 

 corallines, sponges, bryozoa, calcareous algse, and other reef-building 

 organisms, which grew practicalh^ where they are now found. With 

 these, but more abundantly on the flanks of the reef, are found the 

 remains of innumerable crinoids, brachiopods, and other attached organ- 

 isms; while remains of vagrant types, such as mollusks and Crustacea, 

 occur in every part of the reef, frequently in great profusion. On its 

 flanks, where the reef was constantly attacked b}^ the waves, large masses 

 of broken coral occur, which are more or less worn and embedded in 

 the coral and crinoid sand which forms the chief enclosing mass of the 

 reef. This sand, which is of varying degree of fineness, is perfectly strati- 

 fied, the beds dipping awa}^ from the reef at high angles in all directions, 

 but soon falling to a low angle of dip. Around the reef occur interstrati- 

 fications of the bedded sand and the organic masses, the latter not infre- 

 quently extending far out over the bedded rock, between which and the 

 next overlying bed they form a dividing film. Away from the reef we 

 often find rock of the finest lime mud, which settled in the quiet and 

 deep water at a distance from the source of the material. 



Comparison with modern Coral Reefs 



A comparison of these ancient reefs or knolls of coral rock with those 

 of modern time shows close analogy. In modern reefs the clusters of 

 growing corals " are scattered like tufts of vegetation in a sand}^ plain,"* 

 and surrounding and connecting the lens-shaped reefs is the coral sand, 

 which is often entirely without recognizable organic remains. In the 

 neighborhood of the growing reef the rock is often a breccia or con- 

 glomerate. 



Except where a reef rises from great oceanic depths, the slopes are 

 frequently much below 10 degrees. This is particularly the case in the 

 Bermudas, where the slopes are very low. Thus the following are given 

 for Hamilton : t 



Westward, for the first 8.9 kilometers, the slope averages 21 degrees 

 50 minutes; for the next 11 kilometers it averages 1 degree 20 minutes, 

 and for the next 36 kilometers it averages 2 degrees 27 minutes. On the 

 same island the slope to south-southwest is 3 degrees 22 minutes for the 

 first 20.5 kilometers, 3 degrees 8 minutes for the next o.o kilometers, 

 3 degrees 55 minutes for the next 8.0 kilometers, and 4 degrees 9 minutes 



* J. D. Dana : Corals and Coral Islands, p. 174. 

 t Dietrich, quoted bj' Walther. 

 XLIX— Bull. Geol. Soc. Am., Vol. U, 1902 



