STRUCTURE OF THE TURKS ISLANDS 257 



New Providence, and which are so characteristic of all the ridges of 

 the islands of the group. After the formation of the islands came 

 an extensive gradual subsidence, which can be estimated at about 

 300 feet, and during this subsidence the sea has little by little worn 

 away the aeolian hills, leaving only here and there narrow strips of 

 land in the shape of the present islands." 



Before I proceed to give in detail the results of my observations 

 in the Turks Islands some discussion of the views of Agassiz is 

 necessary. He considers that the disintegration of the original land- 

 surface is still in progress; but this would be under present-day 

 conditions, which would be quite other than those which prevailed 

 when the land was being broken up during the movement of sub- 

 sidence, and could not safely be employed in illustration of it. In 

 this connection he regards the strips of land on the Caicos and Turks 

 Banks as representing the last stages in the process of destruction, 

 the original condition of those banks being exemplified in the neigh- 

 bouring large island of Great Inagua, where the whole area of the 

 bank is occupied by land ; whilst the intermediate stage is presented 

 in the island of Andros. Another point is that this investigator 

 lays no stress on the operation of reclaiming agencies at the present 

 time. Here and there a bight or a cove may have been filled in; 

 but the prevailing tendency in our own day, as in the past, is (he 

 holds) one of destruction. However, as he himself shows, a large 

 portion of the island of Grand Turk has been reclaimed by coral- 

 growth at the present sea-level; and in general terms he describes 

 the islands of the Turks Group as formed in part of aeolian rock and 

 in part of shore coral-rock. 



Then again, when he describes the Bahamas as representing the 

 results of the breaking up, during a movement of subsidence amount- 

 ing to 300 feet, of one huge mass of low land, he could have had in 

 his mind only the western portion of the archipelago. The banks 

 from which the eastern islands rise must have been isolated through 

 the ages. So rapidly do they plunge down into the great depths 

 dividing them that an upheaval of 300 feet would add but little to 

 their size, and would be far from establishing any connection between 

 them. 



Let us take the adjoining banks from which the Turks and Caicos 

 Islands rise. They are united under the sea by a " col " covered by 

 about 250 fathoms of water, and lying a few miles to the south of 

 them. This " col " is the head of a deep submarine valley, running 

 north between the two banks along their whole length, which is at 

 least about 1500 fathoms deep opposite Grand Turk and about 2000 

 fathoms where it debouches on the ocean's floor. A subsidence of 

 300 feet (fifty fathoms) could not have effected the separation of 

 these two banks, seeing that the " col " was some 1200 feet under 

 the sea before it commenced. Moreover, isolated as the two banks 

 are from each other, this " col " is in relatively shallow water as 

 compared with the ocean depths around them. The great hollow 

 that separates them on the south from Hispaniola is 2000 fathoms 

 and more in depth. Similar depths of 2000 fathoms isolate the 

 Caicos Bank from the neighbouring island of Mariguana, and prob- 



