THE OCEAN. 671 



form ; but these serve only to divide one of the great channels, and 

 make a new branch. 



The structure of the formations, made from river and oceanic action 

 combined, has been described in connection with the remarks on deltas, 

 on page 651. Sand-flat formations are made of sand, because the 

 movement of the waves is sufficient in force to carry off the finer 

 material. The stratification, or bedding, is parallel to the general sur- 

 face of the flat, because the successive additions are laid over this sur- 

 face ; consequently, the bedding will be horizontal, or nearly so. The 

 sand-beds, where in shallow waters, and washed over by the tidal cur- 

 rents, have often the layers obliquely laminated (Fig. 61, p. 82) ; 

 and, as the in-going tidal current moves with the greatest force, this 

 lamination usually dips toward the direction from which this current 

 comes, or rises in the opposite direction. The deposits in shallow 

 waters off a coast are usually of sand or mud — river detritus and the 

 detritus from the wear of the sands and pebbles of the sea-shores being 

 the material of which they consist. They have sometimes great breadth, 

 as over the submerged plateau off the coast of New Jersey, which is 

 fifty to eighty miles wide. And, as the bottom varies inappreciably 

 from horizontality, the stratification or lamination will be equally hori- 

 zontal. Where there are strong flows of the tide between islands and 

 the mainland, or among groups of islands, the material may be in part 

 pebbly ; and oblique lamination may be a feature of the beds. Over in- 

 terior oceanic basins, as well as off a coast in quiet depths, fifteen or 

 twenty fathoms and beyond, the deposits are mostly of fine silt, fitted 

 for making fine argillaceous rocks, as shales or slates. When, however, 

 the depth of the ocean falls off below a hundred fathoms, the deposi- 

 tion of silt in our existing oceans mostly ceases, unless in the case of 

 a great bank along the border of a continent. 



As heretofore stated, the material of the bottom of the submerged plateau, above 

 referred to, outside of a depth of one hundred feet, consists at surface largely of 

 Rhizopod shells. Off southern New England, at depths between 300 and 550 feet, 

 from a region southeast of Montauk Point to that southeast of Cape Henlopen, the 

 soundings, according to Bailey (Smithsonian Contrib., ii., and Am. Jour. Sci., II. xvii. 

 176, xxii. 282),consist chiefly of these shells. At greater depths, beyond the limit of the 

 plateau, Pourtales found almost a pure floor of Rhizopods (Trans. Am. Assoc, for 1850, 

 84, and Rep. Coast Survey for 1853 and 1858); and the facts have been confirmed by 

 later investigation. The species are deep-water forms, differing thus from those of the 

 New Jersey Cretaceous beds. Pourtales observes, in a letter to Professor Bache (dated 

 May 17, 1862), that, along the plateau between the mouth of the Mississippi and Key 

 West, for two hundred and fifty miles from the mouth, the bottom consists of clay, 

 with some sand and but few Rhizopods; but, beyond this, the soundings brought up 

 either Rhizopod shells alone, or these mixed with coral sand, Nullipores, and other 

 calcareous organisms. 



As microscopic life abounds in harbors where rivers make frequent depositions of 

 sediment, the presence of a considerable proportion of Rhizopods is consistent with an 

 annual increase of the plateau from sedimentary depositions. 



