DEPARTMENT OF MARINE BIOLOGY. 197 



Investigations Regarding the Calcium Carbonate Oozes at Tortugas, and the 

 Beach-Rock at Loggerhead Key, by Richard M. Field, Museum of Com- 

 parative Zoology, Cambridge, Massachusetts. 



While enjoying the privileges of the laboratory of the Carnegie Institution 

 of Washington, situated at Loggerhead Key, Tortugas, Florida, I had excel- 

 lent opportunities for studying the conditions of bottom accumulation of 

 calcium carbonate ("drewite") in the shallow lagoons and channels between 

 the reef-flats. The weather was remarkably clear during the earher part of 

 my stay, so that the bottom could be studied over wide areas and samples 

 collected with comparative ease. 



These submarine calcium-carbonate deposits, heretofore called "coral 

 muds," the precipitation of which Drew ascribes to the action of marine 

 bacteria, are of particular geological interest. WTiether or not they owe their 

 origin to the action of bacteria, they bear a striking resemblance in chemical 

 and physical properties to certain exceedingly fine-grained and unfossiliferous 

 limestones of the Lower Paleozoic. I have particular reference to the purer 

 limestones of the Stones River group in the Appalachian province. 



I have studied the hmestones of this group from New York to Tennessee 

 and find that they are particularly characterized by such intraformational 

 structures as desiccation-fractures, i.e., "sun-cracks," "mud-cracks," ripple- 

 marks, and glomerates. The origin of the latter I have attributed to breaking- 

 up of the desiccated, fractured zone and the molding and redeposition of 

 the still plastic phenoclasts by the action of tidal currents or waves. This 

 hypothesis has been substantiated by the action of the fine-grained, bacterially 

 precipitated calcium-carbonate mud from Tortugas, which was discovered to 

 be of a particularly plastic nature, rapidly developing desiccation-fractures 

 when exposed to the air, and capable of producing all of the inorganic intra- 

 formational structures which I had previously studied in the Stones River 

 group. 



Experiments have proved that this plastic carbonate ooze hardens very 

 rapidly when exposed to the air. Desiccation-fractures are not destroyed 

 when the surface bearing them is flooded with salt water mixed with fresh 

 ooze. The ooze filters in between the cracks in the older, sun-baked surface, 

 and thus a series of superimposed, mud-cracked zones can be formed experi- 

 mentally which, in cross-section, are in every way similar to those in the 

 Stones River formations. Furthermore, the phenoclasts resulting from the 

 desiccation of the ooze may be readily shaped into pebble-Uke forms, which 

 upon redeposition and litliification would have the appearance of a true basal 

 conglomerate, in spite of the fact that they had been formed almost simul- 

 taneously, geologically speaking, with the deposition of the primary ooze 

 itself. The significance of this fact, when appHed to the study of physical 

 evidences of disconformities within the carbonate rocks of the Palaeozoic 

 formations, can not be overestimated. 



The general geological aspects of the Stones River limestone point rather 

 strongly to conditions of sedimentation similar to those of the coral-reef 

 latitudes of the south Atlantic, and especially off the southern coast of Florida 

 and the west coast of the Bahamas. 



The "beach-rock" or "coquina" which is apt to encircle the shell-sand 

 keys of the Tortugas group has already aroused some discussion as to its mode 

 of origin. I have studied the "beach-rock" at Loggerhead Key, and although 

 I have not yet fully proved my premises by experiment, it appears as if the 

 phenomena can be accounted for as follows : 



The rock is obviously formed from the same material as the key, except 

 that in the case of the former the shell-sand is more or less loosely cemented 



