38 bulletin: museum of comparative zoology. 



Some of the shallower parts of the bank are practically impassable 

 even for the smallest boats, owing to the existence of extensive stretches 

 covered by shifting sand bores more or less exposed at low water, when 

 they drift like seolian sand in the direction of the prevailing winds, or are 

 run into more or less broken ridges parallel with the direction of the 

 short seas breaking over the flats. Such tracts on the Great Bahama 

 Bank are formed on the northeast part of the bank extending ten miles 

 to the westward of the Berry Islands, where we find the water on the 

 bank varying from a quarter fathom to a fathom to the westward of the 

 cays, and extending to a line running diagonally across the bank from 

 the Northwest Channel to Great Stirrup Cay (Plate XII. Fig. 4). An- 

 other track extends to the north and northwest of Andros, about fifteen 

 miles to the westward of the Joulter Cays (Plate XI. Fig. 3). A similar 

 small patch runs parallel with the southeast end of Andros for a distance 

 of nearly ten miles, extending westward from Curley Cut Cays and rising 

 from the great sand flat, which increases very gradually in depth as we 

 go west, having a depth of only two fathoms fifteen miles southwest of 

 the cays and three fathoms at a distance of twenty miles in the same 

 direction. Extensive bores also occur to the eastward of the Beminis, 

 and a belt of sand bores varying in width from one to five miles extends 

 from eastward of Gun Cay to South Riding Rock, a distance of more 

 than twenty-five miles. 



There are also a number of such bores on the Mackie Bank, to the 

 eastward of the Beminis (Plate XII. Fig. 2), and between that and the 

 Northwest Passage another extensive tract, forming a bank of about ten 

 miles by eight, traversed by numerous sand ridges, carrying from one 

 and three quarters to two fathoms and from two and a half to three 

 fathoms between them. From Orange Cay in a northerly direction as 

 far as the latitude of the Beminis we find a number of isolated patches or 

 banks of sand, held together by the masses of a species of Thalassia grow- 

 ing upon them. About twenty-five miles to the south of Orange Cay 

 there is a narrow belt of sand ridges running nearly parallel with the 

 100 fathom line for a length of about eighteen miles. These banks and 

 bores are limited to the area north of the great marl deposit to the west 

 of Andros, which extends from a line running west a few miles north of 

 Billy Island to from five to ten miles south of South Bight, the western 

 limit of which varies from three to five miles from the 100 fathom line. 

 Outside of these limits we again find the aeolian sand, more or less modi- 

 fied by the fragments of coralline alga? or of Invertebrates which once 

 lived upon the banks. Algse also flourish upon the white clay or marl 



