498 G. H. DKEW. 



Islands and then trends round in a N.W. direction ; beyond the 

 100-fathom line the depth increases with moderate rapidity until 

 depths of from 1000 to 1400 fathoms are reached. To the E., KE., 

 and N., as far as the coast of Florida, the water is shallow, the 

 soundings showing from 20 to 30 fathoms in most places. Beyond 

 the 100-fathom line to the southward the influence of the Gulf 

 Stream begins to make itself felt, though the region of maximum 

 current velocity here lies nearer the coast of Cuba. The Tortugas 

 Keys are of purely coral formation ; they consist entirely of broken 

 shell and coral sand, and no soil is present ; the greatest elevations 

 are the hurricane ridges, which are not more than 15 feet above sea- 

 level, and during a hurricane the islands are sometimes completely 

 submerged. There is no vegetation on the smaller Keys, but Logger- 

 head Key, on which the Carnegie Laboratory is situated, is partially 

 covered with a growth of bushes and coarse grass. There is no fresh- 

 water supply on the islands. 



From these considerations it is obvious that the risk of contamina- 

 tion of samples of sea-water, taken a few miles from the Keys, through 

 land bacteria is very small, and that such samples may be taken as 

 being truly oceanic. 



The motor-yacht Anton Dolirn, and smaller motor-boats, made 

 the collection of samples an easy matter, and the well-equipped 

 Laboratory made possible fuller investigations than those attempted 

 in Jamaica. 



A number of cultures were made in Gran's medium under conditions 

 exactly comparable to those made at Port Eoyal, and the rate at 

 which the process of denitrification proceeded was observed. The 

 results agreed almost exactly with those obtained at Port Royal, so 

 need not be described in detail. It thus seems that the denitrifying 

 power of the bacteria in the seas around the Tortugas is the same as 

 that of those around Jamaica. 



Cultures were also made on various solid media, and pure cultures 

 of the various species of bacteria were isolated by plating in Petri 

 dishes with Peptone Agar. Samples of surface water taken from 

 various positions round Tortugas as far as possible removed from 

 influence of the land, and collected on sunny days, gave an average 

 count of 14 colonies per 1 c.c. of sample. Counts of several plates 

 from the same locality, and from different localities, showed a some- 

 what remarkable agreement as to the number of colonies present, 

 the highest count ever obtained being 20 and the lowest 8 per 1 c.c. 

 Allowing for experimental error, this shows great uniformity in the 

 distribution of bacteria in the sea round Tortugas. 



