52 bulletin: museum of comparative zoology. 



Notes on the Geology of Southern Florida. By Leon S. Griswold. 

 (Plates XVII. to XXVI.) 



The Ocean Border. — The southeastern extremity of Florida is pro- 

 tected on the side toward the ocean by an almost continuous line of 

 narrow islands, — the Florida Keys. Between these islands and the main- 

 land thei'e is a continuous shallow water body, constricted for short 

 intervals to a width of a hundred yards or less, but generally some miles 

 in width, even to twenty-five miles. Bars and flats (Plate XXI. Fig. 1), 

 with or without aerial vegetation, divide this large water area into a 

 multitude of small sounds. In many places, at depths of five to twelve 

 feet, the floor of these sounds is rock ; the floor is not smooth, yet affords 

 no anchorage. An idea of the sea floor may be had from Plate XVIII. 

 of the shore at Cocoanut Grove at half tide. (See also Plate II.) 



The Foundation of the Mainland. — On approaching the mainland on 

 Key Biscayne Bay the rock floor conies near the surface, so that speci- 

 mens may be obtained. The rock is no longer elevated reef, but a fine 

 grained oolitic limestone, containing occasional shells and supporting a 

 growth of southern pine. Shore line bluffs and vertical sections ob- 

 tained inland show stratification clearly, and for the most part cross- 

 bedding. (Plate XIX. 1 ) The deepest section obtained, sixteen feet in 

 a well at Cocoanut Grove, showed numerous fragments of coral at the 

 bottom. This would tend to indicate that the oolite covers a reef. The 

 bottom of the well is about at sea level, and variations in the height of 

 the sea water affect the depth of water in the well. This fact is general 

 at Cocoanut Grove. 



If we can assume a broad reef as underlying the oolite, then there is 

 a long narrow uplift along the line of great keys, or there is a line of 

 dislocation just on the west side of these keys, the upthrow being to the 

 east. Analogy with the phenomena of the mainland would favor the 

 idea of folding. 



The Pine Belt. — The rise of the oolite from the sea is gradual, vary- 

 ing up to about four feet in a hundred. Bluffs even ten or twelve feet 

 high occur where conditions are favorable for shore cutting, and there 

 arc old bluff lines now protected by marsh. The bluif condition pre- 

 vails for some miles south of the Miami River; north and south of this 



i This bluff is a most distinctly marked seolian rock exposure, with character- 

 istic knife edge stratification. —A. Agabsiz. 



