562 PROCEEDINGS OF THE AMERICAN ACADEMY. 



"Chub" and "Western Blue," on the west; "Mills Breaker" and the 

 " Ship Channel," on the east. On the southeast, running about parallel 

 with the land and at a distance of one to three miles, is a line of de- 

 tached ledges. The great " lagoon," enclosed by the boundary reef, is 

 of rather uniform depth, from five to ten fathoms, but is broken in sev- 

 eral localities by great numbers of shoals and ledges, while the Islands 

 themselves enclose several large sounds, which are in more or less free 

 communication with the sea. The rise and fall of the spring tides at 

 Ireland Island (Map 4), as given on the admiralty charts, is four feet, — 

 only about eight inches more than the vertical height of the tidal wave. 

 The inflow of water into the great lagoon takes place over the whole 

 boundary reef, so that it forms no strong currents, except through the 

 cuts. On the bank as a whole, with the exception of one or two inlets, 

 — for instance the Flatts Inlet into Harrington Sound (Map 3), — the 

 tidal currents are too slight to cause any important transference of mate- 

 rials. Bermuda lies nearly one hundred miles south of the southern 

 margin of the Gulf Stream, and there is no general sweep of water 

 across the bank, while the land crescent forms an effective barrier against 

 any surface currents which might follow the prevailing southerly winds. 

 A good account of the probable geologic history of Bermuda — a grad- 

 ual upbuilding of a volcanic cone by the accumulation of organic debris, 

 followed by a subsequent subsidence — has been given by Agassiz ('95). 

 No outcrop of volcanic rock has ever been found, but it is generally 

 agreed that the core of the bank is of that nature, although buried under 

 a thickness of limestone which is estimated by Agassiz as at least four 

 hundred and fifty feet. The limestone of which the islands consist is of 

 aeolian origin, being f )rmed from wind-blown sand by a consolidation 

 due to the action of water percolating through it, a process which is 

 still going on in the dunes at Tucker's Town (Map 2). This aeolian 

 rock is underlaid in places by the much liaider beach rock, which is now 

 being formed at Great Turtle Bay (Map 4), on the south shore. "Whether 

 there was ever any considerable elevation of the bank above the level of 

 the sea, or whether the dunes and sandhills were formed near that level, 

 as maintained by Agassiz, is of no great moment in this connection, but 

 it is certain that at one time most of the surface of the bank was dry land 

 and that it has subsequently subsided until only the narrow chain of 

 islands remains above water. An outcrop of this ancient land remains 

 in the " North Rocks." Agassiz has described the formation of the 

 'lagoons as resulting from the breaking down of the walls between natural 

 depressions owing to denudation combined with the destructive action of 



