departme;nt of marine biology 157 



florida studies. 



GEOLOGY AND GEOLOGIC PROCESSES. 



While anchored in Biscayne Bay and during the cruise from Miami to 

 Key West, the opportunity was used to examine many keys not previously 

 inspected. These included Ragged Keys and Sands Key, Tea Table and 

 Indian Key, and the smaller keys, Duck and Pigeon, between Key Vaca and 

 Bahia Honda, all of which are largely composed of elevated coral-reef rock ; 

 elevated reef rock was also seen on Bahia Honda Key. The larger keys and 

 some of the smaller ones between Big Pine Key and Key West are com- 

 posed of Key West oolite. Summerland, Sugar Loaf, East Rockland, Rock- 

 land, Boca Chica, and Stock Island were examined and found to be com- 

 posed of o51ite. Most of the small keys north of the main keys and some of 

 those to the south are mangrove keys. The Key West oolite, however, ex- 

 tends under the sea to the north of Key West and Stock Island. These data 

 are in accord with those previously published on the kinds of rock forming 

 the Florida keys. 



During this cruise the mangroves of the regions were again studied, and 

 important accumulations of mangrove peat noted at numerous localities. 



The collection of bottom samples was continued, and specimens were 

 obtained westward of Newfound Harbor Keys to Marquesas Keys. The 

 attempt to find oolite in process of formation was continued, and although 

 the material bearing on the problem has not yet been studied in detail, the 

 following observations will be reported. It was stated in giving the account 

 of the Bahama expedition that as the efforts to find oolite forming directly 

 through precipitation and segregation of particles had negative results, it 

 seemed necessary to attribute the oolitization of the calcareous precipitates 

 to a secondary process. 



Attention was then directed to discovering the process by which the 

 rearrangement of the particles was effected. As it was surmised that the 

 process might be associated with alternate wetting and drying, due to the 

 rise and fall of the tides, specimens of mud were collected in Marquesas 

 Lagoon and taken to Tortugas for observation and experiment. Although it 

 appears that the effect of wetting and drying in converting the mud into 

 oolite may be dismissed from serious consideration, observations, perhaps 

 of importance, were made. The muddy bottom of this region gives off 

 large quantities of gas in which HjS is important, and it is present in all 

 samples of mud. It was noticed that gas-bubbles* had formed in the sam- 

 ples that had been kept wet. The size of the bubbles varies greatly, but 

 many range between 0.3 and 1.5 mm. in diameter, and remain embedded in 

 the mud. Small particles of calcium carbonate collect around the peripheries 

 of the bubbles and form crusts. These crusts simulate oolite crusts, in some 

 instances several, two or three, concentric shells were observed. The in- 

 teriors of the bubbles may be hollow, or there may be an included nucleus 

 of a single grain or several grains of solid material. Furthermore, in some 

 instances the included gas diffused outward through the surrounding crust 

 without rupturing it, permitting a contraction of the coating without its los- 

 ing its spheroidal or ovoid form. The matrix in which the incrusted bubbles 

 are embedded varies in texture as the texture of the mud varies, which may 

 be a putty-like paste, or contain a large or small proportion of fragmental 

 material. The bubbles in some specimens were so numerous as to render 

 the entire mass vesicular. Specimens illustrating the phenomena described 



* This gas has not as yet been chemically analyzed. 



