109 



The first division takes in the top of these ridges, and extends 

 horizontally to various distances, according to the configuration 

 of the Shoal. It is composed of sand, mostly quartzose, con- 

 taining very little Feldspar, with some grains of Hornblende 

 very much worn, but no animals. This sand, although very fine, 

 is remarkable for its almost stony hardness. It has been a ques- 

 tion to what this should be attributed. Some have thought that it 

 may be owing to a cement combined with it, but on being dried it 

 is found to lose its compactness. Mr. Desor stated it to be the 

 opinion of Lieut. Davis, of the U. S. Coast Surveying service, 

 that it is produced by the hammering action of the waves. 



From the second division, which is directly below this, the 

 dredge brings up nothing but broken shells, exhibiting marks of 

 the powerful action of the sea. It is the opinion of Lieut, Davis, 

 that those species in this division, which have not come from the 

 deeper water at the base of the sand ridges, are brought from 

 the neighboring shore by the tides ; but as yet this needs confirm- 

 ation. This division extends to a vertical depth of from three 

 to five fathoms. 



The third division, next below the second, contains pebbles, a 

 few barnacles, and some species of Memiranipora. 



The fourth division, at the bottom of the interval between the 

 ridges, abounds in animal forms. Every stone is entirely covered 

 with corals or barnacles. It is worth remarking that the species 

 here existing are not peculiar to this place, but are found under 

 other conditions nearer the surface of the water. Thus, the 

 Natica heros^ found on Nantucket Shoals at a depth of from tej 

 to fifteen fathoms, may be obtained on Chelsea Beach. The 

 common Star-fish, found at the depth of twenty-five fathoms 

 on Nantucket Shoals, exists at Beverly in shallow water. Other 

 species, as Cardita horealis, for instance, species of Buccinum 

 and Astarte, are subject to similar conditions of distribution. 

 These facts are at variance with the opinion entertained by some, 

 that each marine species has its district at a fixed depth below 

 the surface. It may be true of some, however, which are found 

 in brackish or fresh water. The pressure of the water prevents 

 the existence of animals at a very great depth. The beating of 

 the waves, on the other hand, limits their range upwards. On 

 Nantucket Shoals this is very powerful, and is supposed by Lieut. 



