AGASSIZ: BERMUDAS. 269 



of the bank, scouring the lagoon to the depth and dimensions it has 

 now attained. There is nothing to show that the depth of the lagoon of 

 Hogsty Reef is due to subsidence, or that the rate of growth of the reef 

 was synchronous with it, and thus formed the outer rim of the lagoon. 

 On the contrary, judging by analogy and by the conditions existing in 

 the Bahamas and Bermudas, we are led to infer that the lagoon of Hogsty 

 has been formed by a mechanical process, that it is due to the action of 

 the surf acting as an immense force pump driving the water over the 

 weather face of the reef 1 out through the lee opening of the lagoon, or 

 the openings of the sides of the ring, much as the diminutive serpuline 

 atolls and crescent-shaped reefs or barrier reefs have been formed off the 

 south shore of the Bermudas. 



POT-HOLES. 



Plates XXIX., XXX. 



Roots and stems, after being decomposed, may form branching cavities 

 which if filled with stalagmitic matter would give rise to columnar struc- 

 tures. Such formations can be traced in the more recent dunes of the 

 Bahamas and Bermudas, and of the Sandwich Islands. There is a type 

 of pot-hole which imitates these structures 2 and has been confounded 

 with the branched bodies, but which I do not believe to be organic struc- 

 tures at all. 3 They are the so called palmetto stumps, of which Rice has 

 given an excellent, description, 4 and which I imagine to be mechanical 

 structures of a similar origin with the serpuline reefs (Plate XXX.). 



To the north of the Devil's Hole, on the road skirting the east shore of 

 Harrington Sound, we find a flat ledge of aaolian rocks which is literally 



1 That water falling from a small height does thus excavate deep holes at the 

 foot of falls is well known. Any country ditch dammed by a sluiceway will show 

 this effect. It can be seen at the foot of every waterfall, and it occurs on the 

 largest scale in the Mississippi, where the scour of the river below New Orleans 

 has since the building of the jetties excavated a depth of between sixty and 

 seventy feet in some cases, — a depth about the same as that of many lagoons of 

 coral atolls. 



2 I cannot agree with Thomson (Atlantic, I. 330) in his explanation of the 

 mode of formation of the pseudo palm stems, who considers them to have been 

 formed on the bottom of caves by the dropping of stalagmite, and thus forming a 

 single or double or dumbbell-shaped stem. 



3 Are not some of the tubes to which Professor Dolley ascribes a vegetable origin 

 merely small pot-holes such as I have figured on Plate XXIX. ' 



4 Bulletin of the National Museum, No. 25, p. 27. 



