﻿THE CURRENTS OF THE OCEAN. 155 



a constant state of motion. The channels, on the other hand, are sheltered by the 

 shoals, as well as defended from disturbance by their greater depth. This double pro- 

 tection secures a condition of permanent rest, requisite for the full development of 

 marine animal life. The pebbles, higher up, notwithstanding that they are liable to be 

 moved, appear to be sufficiently stable to become the base on which the ruder animals 

 build, and the home for a few of the moving species. The dead and broken shells are 

 to be included among those materials that are easily transported by the currents, for the 

 collection of which the shoals serve as nuclei. In harbours and inclosed waters gener- 

 ally, animal life will exist in abundance at a less depth, because these positions are more 

 protected by the land from the effects of the waves. 



It is only with regard to the Nantucket Shoals that the depth varying from seven to 

 twenty fathoms (from 42 to 120 feet) is specified as the most prolific. There is no in- 

 tention of saying that these depths are universally the most abundant. Animal life is no 

 doubt to be found elsewhere in similar quantities and diversities much farther below the 

 surface. The greatest of these depths, however, is shallow compared with the abysses of 

 the ocean, which must be wholly destitute of those classes that derive their sustenance 

 from the bottom. By the help of the foregoing facts, we are able to account for the 

 enormous display of marine fossils found in the earlier strata, and distributed throughout 

 regions of great extent. The shallowness of the sea in those periods and at those 

 places afforded the condition best suited to the fruitful development of animal life. And 

 the changes that have led to the present inequalities of the bottom of the sea appear to 

 have confined in a measure to the shoal formations of the actual period the homes and 

 the support of that marine animal life which was formerly more widely diffused. 



This, then, appears to be one of the grand results of the operation of the tidal laws, 

 as disclosed in this paper. Throughout all periods of geology it has prepared the place 

 suitable to marine animal life. 



Another office performed by the tidal and normal currents of the sea has been to 

 transport and diffuse, in great quantities, the loose materials of the ocean, and to increase 

 the area of dry land by taking off and collecting together in plains the detritus of higher 

 regions, filling up, in this manner, the empty spaces between steep and rugged boun- 

 daries, and giving form and body to the continents. Upon looking back to the early ages 

 of geology, we can assign no date to the commencement of such operations other than 

 that of the origin of the tidal currents ; and these are coeval with the first appear- 

 ance of land above the surface of the ocean. But as all these grand operations, by 

 means of which the physical condition of the globe has been made to pass through its 

 successive changes, are intimately connected with each other, and purposely ordered, 

 and as the prevailing tendency of the present laws of tidal and oceanic current action 



