ORIGIN OF CORAL SAND 347 



of the animal. This is particularly true of crinoid stems and calices, the 

 former by their length constituting an important source of calcite frag- 

 ments. Thus are produced the crinoidal limestones which play such a 

 great role among the Paleozoic sediments. The component particles of 

 these crinoidal limestones vary greatly in size in different beds, but are 

 usually quite uniform in the same bed. The average size of the grain 

 will of course be determined by the size of the stem joints of the pre- 

 dominating species of crinoid. These crinoidal limestones are often as 

 well developed where no reefs are found as they are in the neighborhood 

 of the reefs, where they are often of secondary importance. Thus in the 

 rocks surrounding some of the Devonic coral reefs mentioned crinoidal 

 limestones are not at all common, whereas in many of the coarser beds 

 the grain is much finer than the size of the smallest plate of a crinoid 

 stem, while others are composed of impalpable lime mud. It is me- 

 chanically ground up coral rock, ground often into an impalpable lime 

 mud, that constitutes these limestones. 



It appears, then, from our present knowledge that the Paleozoic reef 

 corals were ground up chiefly by the action of the waves. Walther* 

 finds this to be the case in modern reefs, where rock masses, either foreign 

 or broken from the reef, are rolled about over the corals and shell accumu- 

 lations, grinding them into sand and flour, but in the absence of such 

 blocks he finds that the chief reef destroyers are the fish and Crustacea, 

 together with numerous other organisms which assist in a minor way 

 in destroying the calcareous structures. While foreign blocks are un- 

 known in the Paleozoic reefs, a very effective tool for grinding up shells 

 and the smaller corals was present in the huge coral heads themselves. 

 When a Stromatopora head 4 to 8 feet in diameter was rolled about on 

 the margin of the reef by the waves much grinding up of minor calcare- 

 ous structures must have resulted. The heads of Favosites and of Acer- 

 vularia, which, with the stromatoporoids, constitute the chief reef builders, 

 were likewise effective destroj^ers when rolled about by the waves; and 

 that they were thus rolled about is shown b}^ the worn and broken 

 character of all such masses near the reef margin, and the fact that heads 

 many feet in diameter are found overturned or lying on their sides. It 

 is probable that the crinoidal fragments in the neighborhood of the reef 

 were thus ground up into sand and flour, and so converted into clastic 

 sediment. 



The fine lime flour, the most impalpable product of the erosion of the 

 reef, would, of course, be deposited in the quieter water beyond the reef. 

 Around modern coral reefs the water after a storm is milky for great 



*Einleitung in die Geologie, p. 926, 



