154 BULLETIN OF THE 



speed greater than two or three miles au hour, it appears that the Gulf 

 Stream may have had sufficient velocity to bring about this arrangement 

 of the sands. 



It is true that the an'angeraent of sands at and about Nantucket 

 Shoals is brought about by reciprocating currents caused by the succes- 

 sive movements of the tide, while the movements effected by such a 

 stream as that which flows from the Gulf of Mexico would be more con- 

 stant, or in one direction. Nevertheless, it is easy to see tliat A'ariations 

 in the wind cause even at present a considerable variation in the posi- 

 tion of the Gulf Stream off the coast of Florida. Strong winds trans- 

 verse to the surface of the current allect the flow of the superficial 

 waters, occasionally pressing them in against the shore, and again 

 causing a southward-setting current next the beach line. It is quite 

 possible that the variety of movements of the stream which may be 

 necessary to produce an irregular topogi-aphy have been brought about 

 by such variations in direction and force of the wind. 



The gi'eatest difficulty I find in accounting for the topography of the 

 lake district is to explain the presence in the region of the large amount 

 of sand which has been shaped into these irregular ridges and hollows. 

 These sands have evidently come from the northward. It is not easy 

 to imagine the way in which they could have come into the control of 

 the Gulf Stream. One consideration, however, may aid us toward this 

 understanding. With the northern margin of the Gulf Stream crossing 

 Florida at the head of the present peninsula, its current would have 

 swept against the northern shore of the Gulf of Mexico. It thus might 

 have gained access to extensive deposits of sand, which had been accu- 

 mulated in the shallows along that shore. These sands it might have 

 borne onward until it brought them upon the Florida ridge. Tlie ex- 

 istence of a similar action is fdund in the movement of the sands against 

 the eastern coast since the last upheaval of the peninsula. The shore 

 from St. Augustine has received from the floor of the Atlantic an acces- 

 sion of detritus accumulated on the beach which separates the main 

 shore from the open sea. The amount of sandy matter appears equiva- 

 lent to more than one twentieth of that contained in the sand-hills of 

 the lake district. Within the limits of Florida this recently formed 

 sand barrier has a length of about four hundred miles, and an average 

 width of about three miles, and a probable average thickness of about 

 one hundred feet. It is therefore equivalent to a strip having a length 

 about twice as great as that of the lake district, and a rather greater 

 thickness of material. As the lake district averages not more than 



