departme;nt of marine biology. 133 



As is well known, west of Bahia Honda the trend of the main keys is not 

 parallel to the Florida coast, but transverse to it, running from northwest to 

 southeast. The following keys, excepting the southeastern corner of Big 

 Pine, are composed of oolitic rock : Xo Name Key, Porpoise Key, Big Pine 

 Key, excepting the southeastern corner; Ramrod Key, Summerland Keys, 

 Knock 'Em Down Keys, Torch Key, Sawyer Key, and Key West Island. 

 No coral bowlders were seen on Key West. Prof. Louis Agassiz, however, 

 reports that their lower strata consist of coral bowlders and coral breccia, as 

 was ascertained during the excavation preparatory to the foundation of the 

 fort of Key AVest. None of the keys between Key West and the Tortugas 

 were examined. 



Loggerhead, Bird, and Garden keys of the Tortugas consist principally of 

 comminuted shells and corals, which are sometimes considerably indurated. 



The Florida keys, in the main line, between the point of land bounding the 

 eastern side of the Bay of Biscayne to Key West, present three different 

 types. First, the sand keys to the north, which comprise Virginia Key and 

 Key Biscayne: second, the keys composed primarily of elevated coral-reef 

 rock extending from Soldiers Key to the southern end of Big Pine Key; 

 third, west of Bahia Honda the keys are composed mostly of an oolitic lime- 

 stone. The Tortugas are lithologically different from any of the types just 

 recorded. 



Within the main line of elevated coral-reef keys and transverse to them 

 are shoals which in some instances are built to the surface of the water, and 

 form mangrove keys. It appears that these shoals are formed in the follow- 

 ing way : Between the separate keys of the main line are channels out and in 

 which the ocean currents flow. The currents in these regions are very 

 strong. Behind any given key is a region of slack-water, and therefore one 

 of deposition. Shoals consequently are built up behind the keys, and are 

 elongated in a direction transverse to their axes. In the course of time these 

 shoals approached sufficiently near to the surface of the water for floating 

 young mangroves to catch on the bottom. The mangroves grow rapidly, 

 establishing a mangrove key, and then become a constructive agent, ultimately 

 bringing the land surface of the key above the surface of the water. This ap- 

 pears to have taken place in both Virginia and Lignum Vitse keys. 



The mode of origin of the Florida oolite has been the subject of consider- 

 able discussion and divergence of opinion. My opinion is that it has been 

 formed as a water-laid deposit, probably behind a seaward barrier. The 

 reasons for this opinion are that numerous marine fossils are found in the 

 oolitic material, the two valves of bivalve mollusks are frequently in place, 

 showing no damage by attrition, and fossil corals which exhibit no indications 

 of having been rolled or waterworn were found. The marine fossils found 

 in the oolite had evidently lived in the water during the formation of the 

 oolite. 



On the surface of Big Pine Key original mud-cracks, formed by desicca- 

 tion, were observed and photographed. 



I also made a short expedition to Picquet Rocks, Gun and Cat keys of the 

 Bahamas, in order to compare the geological formations of southern Florida 

 with those of the Bahamas. 



