164 FLORIDA GEOLOGICAL SURVEY-13TH ANNUAL REPORT

narrow barrier-beaches, with lagoons one to five miles wide between them and the mainland. On these beaches the wind has piled up low sand dunes, rarely exceeding 10 or 15 feet in height, which seem to be moving very little at the present time. (Dunes are not as well developed in Florida, or anywhere in the tropics, apparently, as they are north of latitude 40', perhaps because in our climate the vegetation covers the sand too quickly for the wind to disturb it much. The wind has considerable force on the east coast, however, as is indicated by the pines leaning inland at an angle of ten degrees or more in many places.)
 A mile or two back from the shore, at many places along the east coast and also near Cedar Keys, Bayport, and probably elsewhere on the west coast, are old dunes of thoroughly leached white sand, which must have been formed at a time when the land stood a little lower and the peninsula was narrower, for dunes do not seem to be forming at present more than half a mile from the outermost beaches. The absence of such features farther in the interior would seem to indicate that the land has not been depressed much below its present position for a very long time; long enough for the wind to level any dunes that might have existed and for the salamanders and other animals to mix the pure sand with the darker sub-soil.*
 Other shore features. The absence of barrier beaches along the Gulf hammock coast has been commented on in the description of that region. It seems to be correlated with the very gentle slope of the ocean bottom along there, which keeps the waves from beating on the shore just as if there was a barrier beach a few miles out; but just why that type of shore with a minutely irregular marshy border, should be confined to the Gulf hammock region is an unsolved problem. Very likely if there was as much wind on the Gulf coast as on the Atlantic coast the shore would be dif ferent; but there is evidently not, for the pines grow perfectly erect near the Gulf coast, instead of leaning inland as most of them do on the other side of the peninsula.

 *The many patches of scrub (described farther on under soils and also under vegetation) in the lake region are thought by some to represent old dunes, but in many or most cases their topography seems to preclude any such explanation.