﻿154 GEOLOGICAL ACTION OF 



here also exist the smaller trihes that supply their food. It is only necessary to refer to 

 George's and Newfoundland, Cashe's and Jeffries Banks, &c. Nearer the coast the 

 smaller banks are resorted to habitually by fishermen, as the places most favorable to their 

 pursuits. In all these shoal formations, however, there is a principle of distribution de- 

 pending upon the relative depths of the water. 



It has been ascertained by Mr. Agassiz, in his researches on the southern coast of 

 Massachusetts and among the Nantucket South Shoals, that a depth varying from seven 

 to twenty fathoms is, in this part of the open sea, most suitable to the existence of those 

 classes of animals that either adhere to the materials of the bottom, like polypes, or that 

 move always on the ground, not having the power to swim, as the radiata and worms. 

 On the ridges or highest parts of the actual shoals no animals are found. They are 

 marine deserts, in which no life, either animal or vegetable, can be supported. 



In passing from the summit of one shoal through the intervening channel to the sum- 

 mit of another, the following facts have been repeatedly observed by Mr. Desor and my- 

 self.* The top of the shoal, which consists of fine sand, has always been found wholly 

 destitute of the traces even of life. Upon descending a little, the dredge brings up from 

 the side of the shoal, and near its base, large quantities of broken shells, which appear to 

 cover the bottom in some places to a considerable depth. These shells are clean and 

 water-worn, and have no polypes or animal remains attached to them. Still lower 

 down, the freight of the dredge will consist of pebbly stones, rounded and smooth, and 

 a few animals. The pebbles may have polypes (membranipora) on all parts of them, 

 showing that they are liable to be moved by the agitations of the sea. From the lowest 

 part of the channel or valley the dredge brings up stones of a larger size and more irregu- 

 lar shape, having rough and uneven surfaces, and covered on one side chiefly with 

 polypes and barnacles, and, together with these stones, a great number of animals, such 

 as echini, starfishes, worms, crabs, and numerous bivalves and univalves. The same suc- 

 cession will be repeated in the reverse order, on passing to the summit of the next shoal. 



These facts may be thus explained. The shoals themselves are composed of ma- 

 terials not adapted for the support of life ; and they rise so near the surface as to be 

 subjected to the constant destructive action of the waves, which break upon them in ■ 

 all but the calmest weathers. Although their average height remains the same, yet 

 the loose materials of which they are constructed are easily transposed, and must be 

 occasionally increased and lessened by the alternate deposits of the tidal currents, 

 and the destructive agitations of the sea in heavy storms. The shoals are, in short, in 



* These facts were presented by Mr. Desor to the Scientific Convention recently held at Philadelphia. 



