348 A. W. GRABAU — PALEOZOIC CORAL REEFS 



distances, owing to the presence in suspension of this lime-rock flour. 

 Agassiz has noticed this fine sediment in suspension at a distance of 12 

 to 20 kilometers from the reef. After a prolonged storm, 4 to 5 centi- 

 meters of coral mud were laid down between two tides.* 



Bryozoa Reefs 



Another type of reef common in some of the Paleozoic rocks has 

 attracted considerable attention. These are lens-shaped masses of com- 

 pact argillaceous limestone embedded in stratified limestones or shales. 

 The}^ are found in the Siluric of western New York, and have been de- 

 scribed by Ringueberg,t Clarke, J Grabau,§ and Sarle.|| They consist of 

 a structureless mass of compact argillaceous limestone, in which organic 

 remains are commonly abundant. These lenses, as pointed out by 

 Sarle, are Bryozoa reefs, around and on which organisms of varying 

 types found a congenial feeding ground. The fine calcareous sand 

 caught and held by the bryozoan fronds helped to build up the reef 

 rock and buried the remains of the organisms feeding on these reefs. 

 Some of the " Klintar " of the coast of Gotland appears to be of this tj^pe 

 of reef. 



Classification of Reef Limestones 



genesis of the limestones 



When we now consider the genesis of the limestones which enter into 

 the composition of a coral reef, we notice at once that a division into 

 two groups is possible — the organic and the clastic. The organically 

 formed limestone is seen in the reef itself, where the coral masses remain 

 more or less in the position in which they grew. On the margins of the 

 reef we have the broken coral breccia and conglomerate, while farther 

 away we have the stratified coral sand rock. Finall}^, in the quieter 

 places, is formed the coral silt or coral flour rock. These three t3^pes of 

 clastic limestones may be found in the neighborhood of nearly every 

 coral reef, and they generally form the economically more important 

 portion of the deposit, since they are well stratified and readily quarried 

 for building purposes or for lime. This is particular!}^ the case with the 

 coral and shell sand limestones and those composed of lime mud. 



*A. Agassiz : Three Cruises of the Blake, vol. i, p. 84; quoted by Walther, p. 929. 

 fAm. Naturalist, September, 1882, p. 711. 

 X Report of State Paleontologist for 1899. 



g Guide to Geology and Paleontology of Niagara Falls, 1901, pp. 99-102. 



II Clifton J. Sarle: Reef structures in Clinton and Niagara strata of western New York. Am. 

 Geologist, November, 1901, pp. 282-299, pi. 27-31. 



