SECOND ANNUAL REPORT-SOUTHERN FLORIDA. 
197 
in rock structure and a difference in the direction of the forces which 
have shaped the islands. 
Bahia Honda and the keys east of it represent am uplifted coral 
reef more or less covered with sand and marl, hence their basement 
rock ridges have the trend of the coral patches of the old reef. The 
foundation of the keys west of Bahia Honda is an oolitic limestone 
formed from deposits in a broad expanse of shallow water; hence 
there was no original ridge-like upbuilding, no pronounced trend to 
the rock structure. Variations in resistance to erosion have resulted 
in irregularities of the rock surface, which, as along the old reef to 
the east, has been more or less covered with marl and calcareous sand. 
The prevailing direction of the passages separating the keys is due to 
tidal currents, which owe their power to differences in time and height 
of the tides of the Gulf and the Strait of Florida. 
The shaping of the great arc of the keys is the joint product of 
several factors. The old coral reef was built up from the bottom in 
water of a certain depth along a line that had the general direction of 
the southeastern and southern edge of the submarine plateau of the 
Florida peninsula. The curve of this edge had been in turn controlled 
more or less by the eastward flow of the Gulf Stream against the 
probable westerly movement of the prevailing winds. 
The decided differences of surface, bare rock, or rock with a very 
thin veneer of leaf mold, sand, and marl, and the slight differences of 
elevation above high tide result in great differences of vegetation. 
Near the water’s edge and on flats or even rock beaches below 
high tide level grow mangroves, on the beach ridges cocoanut palms, 
not indigenous, flourish. Inland the low marl flats support grasses, 
sedges and a variety of salt-resistant weeds. The higher ground called 
hammock, supports a dense growth of scrubby hardwood trees, button- 
wood, ironwood, madeira, etc., that seldom attain a height of more 
than twenty feet, while on three of the keys, No Name, Little Pine and 
Big Pine, notably the latter, are patches of pine. 
The surface of the rock outcrops along the keys differs from that 
of most of the bare rock in the Biscayne pineland or in the flat lands 
of the west coast between the. Everglades and Cape Romano. It is 
less weathered, hence more even and, except on beach slopes worn by 
spray, has not a jagged appearance. Angular blocks, disrupted by 
tree roots or by temperature changes are scattered over it, but in 
places it is quite smooth over areas of twenty to one hundred square 
yards. Holes and hollows resembling those found in the Biscayne 
pineland and formed in the same way, abound, but the rock surface 
has a look of newness, its major inequalities are not the result of sub¬ 
aerial decay. It is like that of some of the low keys in the Everglades 
to the west of Long Key. 
