AGASSIZ: BERMUDAS. 255 



The Algae, Serpulae, Corallines, Mytilus, and the whole growth which 

 goes to form the serpuline atolls, form hut a thin coating upon the 

 ledges of aeolian material upon which they happen to have grown. 

 Underlying this animal and vegetable coating we find the aeolian rock, 

 which on some parts of the ledge may still be protected by the hard 

 ringing crust so characteristic of Bermudian and Bahamian limestone. 

 The inner parts of the pool or atoll within the raised walls is com- 

 posed of softer material, or of material which has not been protected by 

 animal and vegetable growth from the destructive agency of the sea. 

 The serpuline atolls are aeolian rock ledges which once were a part of 

 the south shore cliffs at the time when the shore line was farther 

 to the southward and had not yet begun to yield to the inroads of 

 the sea. 



The protecting growth of the atoll has little to do with the formation 

 of the wall forming the rim of the atoll ; in some cases it has undoubt- 

 edly grown up perhaps twelve to eighteen inches above the wall itself, 

 but the deep lagoons and steep vertical walls of the serpuline atolls so 

 characteristic of the southern side of the islands have been formed, I 

 believe, by the mechanical agency of the breakers. These diminutive 

 atolls are large pot-holes excavated by the surf and sand, and the varied 

 forms of circular or of crescent-shaped reefs, of barrier reefs, and all the 

 possible modifications one finds on the south shore of the Bermudas, are 

 primarily due to the mechanical action of the sea. All these structures, 

 from a circular or elliptical atoll to a barrier or fringing reef, with all 

 their possible modifications, are due to the action of the surf and the 

 sea in wearing away the surface of the mushroom-shaped rock, which 

 is either softer than the surrounding parts or is not protected by the 

 covering coat of Algae, Corallines, or Serpulae. 



One can off the south shore trace the whole process from the time 

 when the large fragments of shore aeolian rock fall by undermining into 

 the sea, until they are changed by the action of the surf into mushroom- 

 shaped ledges surmounted by pinnacles, and next into the stage when 

 the pinnacle has in turn been undermined and dropped alongside of the 

 ledge to become the holding ground of coral and other growths. The 

 surface of the flat ledge which formed the base of the pinnacle is now 

 freely acted upon by the breakers. According to the nature of the 

 upper crust, and to the extent of protection given to it by the covering 

 coat of animal and vegetable life, the sea acts upon it, and we have hol- 

 lowed out diminutive circular atolls, crescent or horseshoe-shaped struc- 

 tures, as well as the curved, straight, or convoluted or looped vertical 



