A Contribution to the Geologic History of the Floridian Plateau. 119 



Nos. 43, 45, and 46, and these specimens also contained a small propor- 

 tion of quartz. 



The bottom samples show that quartz is disseminated over the 

 entire area of the bottom of Florida Bay ; however, in comparison with 

 the calcareous constituents the proportion of quartz in the southwestern 

 comer of the bay is extremely small. 



GUN AND C.\T KEYS, BAH.\MAS. 



Four specimens, Nos. 6 to 9, were obtained from the sea-bottom 

 and the beaches of these two keys for purposes of comparison with 

 the material from the Florida keys. Specimens Nos. 6 and 7 were, 

 respectively, from the eastern shore of Cat Key, south of the passage 

 between it and Gun Key, and off the eastern shore of Gun Key, north of 

 the passage between it and Cat Key. The material consisted of shell 

 fragments, amorphous carbonate of lime, sponge spicules, and diatoms, 

 also a little aragonite and calcite. Specimen No. 6 contained a Httle 

 quartz which did not pass through mesh No. 80. This material is very 

 similar to that from the region of Key Vaca and Bahia Honda, with 

 perhaps a smaller quantity of quartz. Specimen No. 8 was from the 

 eastern shore of Gun Key, above tide, and No. 9 is beach sand from the 

 western face of Cat Key. This material was similar in composition to 

 that taken from the bottom except no quartz whatever was observed. 

 It should be noted that specimens Nos. 6, 8, and 9 all contained oval 

 grains of amorphous carbonate of lime. The nature of these grains will 

 be alluded to in the comparison of the hthologic specimens from the 

 Bahamas with those from the vicinity of the Miami and Key West oolites. 



SUMMARY OF DATA ON THE MATERIAL OF THE DEPOSITS. 



The material at present being laid down inside of the keys consists 

 mostly of sihca and carbonate of lime. Silica is abundant in the form 

 of sand in the northern portion of Biscayne Bay, it becomes rarer toward 

 the southwest, and is present in small quantities as far as Big Pine Key. 

 Toward the southwest, as the siliceous material becomes rarer, calcium 

 carbonate becomes progressively more abundant, occurring as a flocculent 

 sediment or ooze over practically the entire region from the lower por- 

 tion of Biscayne Bay to the gulf end of Florida Bay. 



