MARINE HVDROGENIC LIME DEPOSITS 335 



regarded as of purely chemical origin. The rapid cementation of 

 lime sands in the deeper sea by the deposition of lime or magnesium 

 carbonate is a further illustration of this phenomenon, as is also 

 the rapid cementation of reef limestones and the sands and rubble 

 on the borders of the reef. Darwin found that the breccias of 

 coral fragments on the outer margins of the reefs were so firmly 

 cemented that it was difficult to chop off a fragment, even with a 

 chisel. The Pourtales plateau, south of Florida, is a hard limestone 

 surface of recent origin. All loose material not cemented by lime 

 deposits is swept off the plateau by the Gulf Stream. Similar sub- 

 marine banks are found between the Canaries and the South Afri- 

 can coast (the Seine-Dacia and Josephine banks), the Campeche 

 banks of Yucatan, the Mosquito bank of the east coast of Nica- 

 ragua and elsewhere. 



What is true of modern coral reefs and banks is equally ap- 

 plicable to older deposits of this character. The cementation of the 

 organic (biogenic) limestone of the reefs of the various geological 

 periods is undoubtedly due to precipitation of lime from the sea 

 water. This, however, is always of minor importance, and the reef 

 deposits must be classed with biogenic rocks. A rapid cementation 

 of detrital lime sands and muds seems to be indicated in many 

 limestone banks. It has been especially described for the Mus- 

 chelkalk (Philippi-45 :./ :?(?), and is suggested by certain features in 

 a number of limestone beds, and is to be explained as due to rapid 

 chemical deposition of lime between the grains of the calcareous 

 clastic material. 



Among the indications of rapid hardening of such banks is the 

 attachment of crinoids by flat, disk-like expansions, which is pos- 

 sible only on a hard bottom ; by the attachment of molluscs and 

 other hard-bottom-loving organisms, and by channelings and bor- 

 ings which may be filled by the immediately succeeding materials. 

 Such rapid hardening makes possible the formation of conglomerate 

 or of brecciated layers which contain fragments of the underlying 

 limestone layer. Such intra formational conglomerates have been 

 described by Walcott from Lower Cambric limestones of eastern 

 New York and Pennsylvania and from \"irginia and Tennessee (57; 

 58:57), and by Wagner from the Muschelkalk of the vicinity of 

 Jena (56; Fig. 5). Here the lower bed represents a worn surface 

 and many fissures which are filled by the material of the succeeding 

 layer. This upper layer is also characterized by a basal conglom- 

 erate with pebbles of the lower rock. That the upper layer was 

 also rapidly cemented is shown by the occurrence of a large En- 

 crinus root and a rich molluscan fauna over its surface. Much care, 



