582 PROCEEDINGS OP THE AMERICAN ACADEMY. 



this connection, and porites, astraeans, meandrinas, and oculinas ; massive, 

 stony forms, too solid to be ground up except by the very heaviest surf, 

 which occurs only on the south shore. I should add that in speaking in 

 general of corals, I have not meant to include the millepores, which have 

 been of great importance in sand-building, as noted by Heilprin ('89). 



The bottom deposits of Bermuda fall into three main types : first, the 

 blue muds ; second, the white marls ; and third, the shell sands. Of 

 course these may be combined in any one locality, but in their features 

 they are tolerably distinct. The blue muds occur only in small, more or 

 less landlocked basins which are cut off by bars or islands from any active 

 circulation of sea water. They seem to be chiefly of terrigenous origin, 

 being the fine detritus washed down by the rains from the surrounding 

 calcareous hills, together with some vegetable remains. They are 

 relatively bare of animal life, except for a few worms and for the sea 

 urchins, which, as in Mullett Bay, may live in them in extraordinary 

 numbers ; and they agree perfectly in their mode of origin with the blue 

 muds of the continental slope. The areas where they occur are so limited 

 that they are of little practical importance and need not be further 

 considered. 



The white marls consist of white calcareous sand so fine as to form a 

 chalky ooze, containing very few living organisms or recognizable re- 

 mains. They occur chiefly in the deep basins of shallow waters in sheltered 

 localities where there are no strong currents, and in the neighborhood of 

 cliffs, ledges, and reefs. . They are formed, as stated by Agassiz ('95), by 

 the " slow trituration of aeolian rock." 



A good example of the origin of these marls is afforded by the Cow 

 Ground Flats, just west of Ireland Island, where there are innumerable 

 banks and ledges, enclosing many small channels and basins, which have 

 a depth of about seven fathoms. For some reason the mechanical de- 

 struction by the action of the water on these aeolian rocks is very rapid 

 here, and the silt thus formed, which is of almost impalpable fineness, is 

 being deposited in large quantities in the protected basins between the 

 ledges. During strong winds the waters over the whole lagoon are 

 often milky with the great quantity of this silt which they hold in sus- 

 pension (Agassiz, ' 95, p. 212). In certain restricted localities, as men- 

 tioned above, the white marl forms almost the entire mass of the bottom 

 deposit, and it is mixed in greater or less degree with the various deposits 

 of shell sand. 



The shell sands are composed of rather coarse fragments, with great 

 numbers of living organisms, with, in every case, a considerable ad- 



