6 



During the months of March and April I explored the Tortugas 

 under the auspices of the United States Coast Survey. The late 

 Superintendent, the Hon. Carlile P. Patterson, placed at my dis- 

 posal a small steam launch, and I hoped to be able to explore the 

 surface fauna of the Gulf Stream, and ascertain the part taken by 

 the innumerable surface organisms in building up the base 

 (plateau) upon which the coral reefs of Florida have been raised. 

 Unfortunately the weather was not favorable for surface and 

 pelagic work, and little could be accomplished in that direction. 

 I have given an account of the work done by myself and my as- 

 sistant, Mr. Fewkes, in Letter No. 5, addressed to Mr. Patterson 

 and published in the Museum Bulletin. To Lieutenant-Com- 

 mander Winn and Lieutenant-Commander Wright I am greatly 

 indebted for the facilities they placed at my command. Captain 

 Scammon, of the Revenue Cutter " Dix," was also authorized by the 

 Treasury Department to assist us as far as practicable. On my 

 way across from Jacksonville to Cedar Keys I had the opportunity 

 cursorily to examine a section of the peninsula. Dr. Smith has 

 described the limestones of the central and western part as belong- 

 ing probably to the Yicksburg age, flanked only on the very shore 

 by the newer postpleiocene limestones. From the rapid exami- 

 nation I was able to make, I am more inclined to consider that 

 some of the younger limestones have been deposited further inland 

 along the sides of the backbone described by Dr. Smith, and 

 forming there elevated coral reefs similar to those now found in 

 the Everglades. But a very careful survey of the Peninsula of 

 Florida is needed to ascertain how far it extended south during 

 the Yicksburg period, and what portion of it consists of the later 

 limestones and is formed by ancient coral reefs. 



Cambridge, Oct. 1, 1881. 



ALEXANDER AGASSIZ, 



Curator. 



