Botanical Ecology of the Dry Tortugas. 113 



tugas, which has also been extensively studied by Vaiighan, is quite 

 rich and has contributed a great deal to this detritus. The atoll is 

 therefore formed by certain currents arranging this loose calcareous 

 detritus. This material above sea-level is unconsoUdated, but below 

 8 feet indurated beach-rock is found. This constituted an older 

 formation for the growth of reef corals. "It is probable," Vaughan 

 says, ''that this rock was sub-aerially indurated and then depressed — 

 I. e., the atoll was outUned pre\dous to coral growth in this region." 

 The whole of the south Florida region was deposited in Phocene times 

 during depression; then in Pleistocene time there was an uplift and 

 again a depression with a succeeding uplift. During the Pleistocene 

 period it is supposed that some of the coral reefs stood as much as 18 

 feet above sea-level. In recent times there has been a depression, 

 which, however, has left the keys slightly higher than they were before 

 the Pleistocene depression. The evidence of these oscillations is borne 

 out by the knowledge of the growth habit, in relation to sea-level, of 

 the reef-forming corals in the Florida region. 



CLIMATIC CONDITIONS OF THE TORTUGAS. 



While no permanent records are available from the Tortugas, the 

 writer assumes that the records from the office of the United States 

 Weather Bureau at Key West, only 68 miles distant, will for this pur- 

 pose be applicable to the cHmate of the Tortugas. The records in this 

 office have been kept since 1871 and are very reliable. 



For botanical consideration the subject of rainfall is quite important. 

 According to the Key West records the total average rainfall is 38.66 

 inches. The precipitation is quite varied and depends largely in sum- 

 mer on tropical squalls which suddenly appear on the horizon and sweep 

 over the islands with a fierce rush of wind and driving torrents of rain. 

 These perhaps last only 20 minutes to an hour. The maximum total 

 precipitation for 24 hours, for the year 1914, for instance, was 4.80 

 inches, which occurred in November of that year. An idea of the varia- 

 tion may be gained by comparing this with the maximum precipitation 

 for 24 hours in January of the same year, which was only 0.31 inch. 

 It may be mentioned in this connection that the greatest precipitation 

 in this region occurs during the months of September, October, and 

 November. As stated above, the average annual rainfall for Key West 

 (and presumably the approximate for Tortugas) is 38.66 inches. By 

 comparing this with the total average precipitation for Miami, situated 

 on the mainland of Florida, 125 miles north of Key West, which is 46.56 

 inches, it is seen that the average is shghtly lower for the southern keys 

 than for the main peninsula. 



The average number of clear days for these southern keys is 151. 

 During the summer months of May, June, and July the writer has 



