42 NOTES ON THE GEOLOGY OF THE BAHAMAS 



Evidence of Subsidence and Elevation 



The facts bearing on the question of subsidence and elevation 

 have already been given, but it may be worth while to briefly review 

 them and give my inferences. 



The caves and old beaches now above the sea plainly show where 

 the level of the water formerly was. The section at Fresh Creek 

 proves, as do -the other cases of elevated corals, that the island has been 

 elevated. As the formation on top of the corals at Fresh Creek is 

 ^olian, it follows that it could have been deposited only above water; 

 and as the caves and old beaches are at least thirty feet above the sea- 

 level and in the .^olian formation, they could not have been formed 

 until the islands had subsided. Hence we are justified in assuming 

 that at some previous time in their history the islands were at about the 

 same level as now. Then followed a period of subsidence of at least 

 thirty feet, during which the caves and old shore-hnes were formed. 

 After this subsidence the islands were elevated to about their present 

 position. It only remains to sum up the facts that bear on the question 

 of the most recent movement in the Bahamas. 



The fact that on the west coast of Andros, where the slope seaward 

 is so exceedingly slight, the soft calcareous mud grows gradually 

 harder and harder as we go inland, indicates to my mind that the island 

 has been recently rising'; for if it were subsiding or had recently sub- 

 sided, we may suppose that time enough would have elapsed since its 

 elevation to allow the calcareous deposit to harden into rock, and then, 

 as the subsidence took place, the surface at the edge of the water would 

 be hard rock, which would finally probably extend under the water as 

 the latter encroached on the land. The depth, close to the shore, of 

 the fine calcareous deposit also points to elevation, for in it I ran a pole 

 nine feet. How much deeper it was I had no means of determining, 

 owing to the length of the pole. Now, had subsidence been taking 

 place, should we find this depth of calcareous mud close to the shore ? 

 It is perhaps possible, and it might be claimed that the greater the sub- 

 sidence, the greater would be the depth of the mud ; but by the time 

 that nine feet of sediment had been laid down it is reasonable to sup- 

 pose that the mud on the shore would have hardened, and then should 

 we not find the water washing against a rocky shore ? 



As we approach the west side of Andros from the interior, the pines 



