274 PAPERS FROM THE DEPARTMENT OF MARINE BIOLOGY. 



usually less than 2 miles from shore, and depths ranging from 800 to 1,000 

 fathoms are only a short distance farther seaward. On the west side there 

 is an enormous flat, which is over 60 sea miles wide along an east and west 

 line, and on it the maximum recorded depth is 3^ fathoms. In a way 

 there is here a great evaporating pan, and a concentration of saline ingre- 

 dients results. This concentration would cause the precipitation of some 

 CaC0 3 even were there no bacteria. It is probable that, especially during 

 the summer months, the temperature of the shoal waters is higher than on 

 the surface of the ocean where the depths are greater. Such an increase 

 in temperature would cause the water to lose C0 2 and produce precipitation 

 of CaC0 3 . Surface agitation of the water would accelerate the loss of C0 2 

 and thereby increase the rate of precipitation of CaC0 3 . 



From the foregoing discussion it is obvious that there are at least three 

 cooperating factors tending to produce precipitation of CaC0 3 , viz: (1) 

 ammonifying bacteria, (2) concentration of salts in solution through evap- 

 oration, (3) expulsion of C0 2 by increase in temperature. As these factors 

 have not been evaluated, a satisfactory solution of the complicated problem 

 awaits further research. 



SHORE SPECIMEN, NORTH OF WEST END OF SOUTH BIGHT. 



(Specimen No. 83 ; see plate 95 for location.) 



This specimen was subject to alternate wetting and drying by the rise 

 and fall of the tide. The mechanical analysis of it is given on page 269 and 

 it is graphically illustrated on plate 94; the percentages of MgCCV and CaC0 3 

 are given on page 270. The percentage of particles of silt and clay size is 

 55.4; that of MgC0 3 , 13.36. According to the mechanical analysis, this 

 specimen groups with specimens 79 and 87; but it is higher in MgC0 3 than 

 any other of the specimens here considered. There has evidently been 

 secondary concentration of magnesia, perhaps due to alternate wetting and 

 drying by the rise and fall of the tides. The specimen from the northwest 

 end of Murray Island, washed up on the reef, 1,700 feet from shore, has 

 7.57 percent of MgC0 3 , 2.175 P er cent higher than the average of the samples 

 taken from the bottom along line I, southeast reef, suggesting that secondary 

 concentration has also taken place in it. Dr. Cushman says regarding this 

 sample (No. 83): "Little of interest in the material. Foraminifera, few, 

 minute, technical species, unlike preceeding (No. 82)." 



OOLITIC SAND FROM GREAT BAHAMA BANK. 

 (Specimen No. 71; see plate 95.) 

 The Great Bahama Bank is remarkable in its topographic features. 

 There are thousands of square miles of its surface over which the water 

 ranges in depth from 6 feet as a minimum to about 21 feet as a maximum. 

 I know no other plain so extensive in area and so small in the range of the 

 relief of its surface. Along the line from Gun Cay to Northwest Passage, 



'Hypothetical combinations. 



