AGASSIZ: BEKMUDAS. 271 



The flat ledge surrounding Xorth Rock presei;its no features diflfereut 

 from those of similar ledges off the south or north shore. Its surface 

 rises -here and there in low ridges, which are the remnants of the last 

 pinnacles to be eroded ; near the edges, and wherever the action of the 

 breakers reaches, it has been dug into so as to form pools, pits, and pot- 

 holes of various depths and shapes, and has been honeycombed in all 

 directions, according to the quality and hardness of the rock and the 

 extent of the protection afforded the rock surfaces by the growth of Algae, 

 Corallines, Serpulae, and other organisms. 



The North Rock Ledge is deeply undercut, and, like many of the larger 

 islets off the main island, its sides are more or less vertical, or steep 

 slopes deeply honeycombed and cavernous, and overgrown with Algae, 

 Corallines, Gorgon ians, Millepores, and massive corals, much as any 

 similar ledge or cliff or patch. The greater part of its upper surface 

 is protected by the hard ringing aeolian rock characteristic of the ex- 

 posed iutertidal spaces. The pinnacles which remain rising above the 

 general level consist of aeolian rock, and the lower base rock seemed 

 to me to differ in no way from similar aeolian rock as modified by the 

 action of the sea in other localities. It is possible that the fossil Cyprasas 

 stated by Rein to have been found at the base of the pinnacles may 

 be only such shells as have been collected from the -serpuline rock 

 in which they had become embedded. In some spots on the ledge it 

 attains a thickness of from twelve to fifteen inches, and is full of boring 

 Mollusks and of shells which have found a foothold in the cavities of the 

 honeycombed rock. The serpuline rock itself often becomes quite hard, 

 and might easily be mistaken for true hard ringing seolian rock, but is 

 readily distinguished from it by the presence of the many sharp pinkish 

 fragments of the tubes of Serpulae. 



The many Chamas and other Mollusks living on the edge of the North 

 Rock Ledge at low-water mark would, if thrown up and embedded in 

 ajolian or serpuline rocks, present all the characters of the so called fos- 

 sils found in the a;olian rocks of the islands of Hamilton Harbor. At 

 Agers Island and on the shores of the other islands of Hamilton Harbor 

 we find at very low tides many specimens of Chama which would have to 

 be carried but little way to become aeolian fossils. Neither Heilprin nor 

 myself found any fossils on North Rock. Heilprin considers the lower 

 portion of the North Rock pinnacles to be unquestionably pits and pot- 

 holes of the inner surface of the beach rock. 



The outer edge of the northeastern part of the ledge is protected by 

 SerpuliB, Algae, and Corallines, forming low vertical walls as well as the 



