258 PLANTS, SEEDS, AND CURRENTS 



ably also from Great Inagua. To the north of the two banks lies 

 the open ocean with a depth of 2000 to 3000 fathoms. 



No soundings in which bottom was obtained are given in the 

 Admiralty chart (1266) for the seas dividing the three great banks 

 stretching south-east from the Turks Islands, namely, Mouchoir, 

 Silver, and Navidad Banks; but it is not improbable that they are 

 connected by deeply submerged " necks." Except probably on the 

 Turks Island side these three great banks rise up from the ocean 

 depths around them. The trench separating them from Hispaniola 

 is 2000 fathoms deep, whilst to the north is the open sea, where 

 soundings of 3000 fathoms have been obtained. To the east this 

 line of banks plunges down in a few miles to depths of over 2000 

 fathoms, which increase as we go eastward some forty miles towards 

 the Brownson Deep, where depths of 4000 to 4500 fathoms occur. 

 From what has been said above it is apparent that the eastern 

 extremity of the Bahamian area, including the Caicos and Turks 

 Banks with the islands rising from them and the Mouchoir, Silver, 

 and Navidad Banks that carry no islands, has been isolated from the 

 rest of the region by deep-sea conditions through the ages. As we 

 proceed westward the connecting seas become shallower and the 

 banks and islands larger; but the process is gradual, and even 

 immediately west of the Caicos Bank depths of 1200 to 1800 fathoms 

 separate banks and their islands. 



I come now to the reasons assigned by A. Agassiz for assuming a 

 subsidence of 300 feet. They are based on the existence of " ocean- 

 holes" in the banks. These holes have been sounded to a depth of 

 thirty-four fathoms (204 feet) ; and it is to be supposed that in 

 estimating the total depression at 300 feet he allowed for the depth 

 of water covering the banks. He considers that they were formed 

 in the seolian rocks, before the subsidence, by the same agencies that 

 produce the present caverns, sinks, blow- holes, etc., in the geolian 

 sandstone of the hills. We learn from his maps that these " ocean- 

 holes " may pierce the banks to a depth of 200 feet and may be 200 

 yards or so in width. I venture to think that this evidence in sup- 

 port of a depression of 300 feet is a little hypothetical. In any case, 

 the decision must lie with the student of land forms and submarine 

 contours, the interpretation of which has become an important 

 branch of geological science in recent years. 



The subject of these " blue " or " ocean " holes has evidently 

 received much attention from American geologists. Dr. Vaughan 

 has recently procured additional information concerning them in the 

 North-west Bahamas, and he writes that " on the assumption that 

 these holes were subaerially formed, parts of Andros must have once 

 stood 192 feet higher than now." Similar but smaller holes, as he 

 states, exist in the Miami coast region of Florida. They penetrate 

 to a depth of over thirty-five feet a rock floor of oolite lying ten feet 

 below low tide, and are regarded as " indicating the presence of 

 solution wells " in that formation (Year Book of the Carnegie Institu- 

 tion of Washington for 1914 and 1915, No. 13, pp. 227-33; No. 14, 

 p. 234). 



This subject raises another important point. An implication of 



