664 DYNAMICAL GEOLOGY. 



effect. Prof. Bache has thus explained the increase of Sandy Hook 

 (the southern cape at the entrance of New York Bay). His obser- 

 vations prove that the current during ebb tide is most effectual in 

 prolonging and shaping the Hook, though the in-flowing tide con- 

 tributes to the result, and the waves aid in giving the hook-like form 

 by bending in the extremity. The Hook has been elongated at the 

 rate of " one-sixteenth of a mile in twelve years," since the time 

 when the first precise observations were made. 



3. Structure of the formations. 



Beach-formations are irregularly stratified, the layers being much 

 interrupted, and varying every few rods or less, as represented in 

 fig. 61 d, p. 93. The layers are either of sand, gravel, pebbles, or 

 stones. In the lower part, where washed by the tides, they often 

 slope seaward a few degrees. Over the area of shallow waters out- 

 side, the deposits in progress consist mostly of fine detritus, either 

 sandy or argillaceous, with rarely pebbles, except near the beach ; 

 and through the greater part argillaceous beds prevail, as shown 

 by soundings. The beds may be uniform over very large surfaces. 

 These regions are fifty to eighty miles wide on the eastern border 

 of North America (fig. 664, p. 441). In the lagoons or bays, argil- 

 laceous deposits are the most extensive; but sand and pebbles may 

 be distributed among them, especially off the mouths of streams. 



As stated on page 612, the material of the bottom of the submerged plateau, 

 above referred to, outside of a depth of 90 feet, consists at surface one-half 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 Hen- 

 lopen, the soundings, according to Bailey, consist chiefly of these shells. At 

 greater depths, beyond the limit of the plateau, Pourtales found almost a pure 

 floor of Rhizopods. The species are deep-water species, differing thus from 

 those of the New Jersey Cretaceous beds. Pourtales observes, in a recent 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. (Bailey, 

 Smithsonian Contrib. ii. ; Amer. Jour. Sci. [2] xvii. 176, and xxii. 282 ; Pourtales, 

 Trans. Amer. Assoc, Charleston meeting, 1850, 84; Reports Coast Survey for 

 1853, p. 83, and 1858, p. 248.) 



Ripple-marks are often made by the waves over the finer beach- 

 sands, where they are low and partly sheltered, and also over mud- 

 flats. The flowing water pushes up the sand into a ridgelet, as high 



