4 SURFACE GEOLOGY. 



upon tertiary deposits, where they are present, or upon older rocks, where they 

 are not. 



Modified Drift. — When drift has been acted upon by waves, or currents of water, 

 the boulders are reduced in size, they are smoothed and rounded, their striae are 

 generally obliterated, and all the materials are redeposited in regular layers, being 

 sorted into finer and coarser deposits, according to the velocity of the currents. 

 These I call modified drift, which constitutes nearly the whole of what usually goes 

 by the name of alluvium, and assumes various forms, according to circumstances. 



In this paper, the term alluvium includes not only modified but unmodified drift, 

 for reasons which will appear in the sequel. 



Sea-Bottoms. — The bottom of the ocean, along the coast, is in many places covered 

 by deposits of sand and gravel, left there seemingly by tidal action, and presenting 

 often numerous ridges and depressions. Often, too, bars are formed across the 

 mouths of harbors, producing lagoons. Hooks, also, are produced, where the cur- 

 rents sweep around headlands. While these deposits are beneath the waters, they 

 go by the name of shoals. If these shoals, bays, and harbors be raised out of the 

 ocean, although they will be exposed to the modifying influence of rivers and rains, 

 their essential characteristics will be long preserved; and my impression is, that 

 these old sea-bottoms may still be traced in many parts of our country, to the height 

 of 1,000 to 2,000 feet above the present ocean. 



Submarine Ridges. — By this term. I intend to designate certain ridges of sand 

 and fine gravel that must have been formed beneath the waters, and yet are dif- 

 ferent from those ridges called shoals, and, perhaps, from any other submarine 

 deposit described by Lieutenant C. II. Davis, in his admirable paper, in the Memoirs 

 of the Academy of Arts and Sciences, " On the Geological Action of the Tidal and 

 other Currents of the Ocean." The great peculiarity of these submarine ridges is, 

 that they slope in two directions — towards the lake or the ocean, on whose borders 

 they lie, and towards the country; a fact which indicates subaqueous formation. 

 The natural ridges around Lakes Ontario and Erie, are a fine example of the phe- 

 nomenon I am describing. (See Charles Whittlesey's excellent paper, Am. Journ. 

 Sci., n. s., X, 31.) Perhaps, also, I may be able to point out one or two examples 

 on the sea-coast. 



Osars. — These are similar ridges, formed beneath the waters, by currents piling 

 up materials behind some obstruction. Their form is very much like that of a 

 canoe turned over. I have not been able certainly to identify any ridges of sand 

 or boulders in our country with the osars which I saw in Europe. But M. Desor, 

 whose opportunities for observation upon this phenomenon have been very exten- 

 sive, speaks of osars as occurring along the shores of Lake Superior. I have 

 marked four on Map No. 1, (Plate III,) in N. H., viz : in Union, at the White 

 Mountain Notch (at Fabyans), and a little south of Conway; but they are of 

 doubtful character. 



I use the terms dune and delta in their common acceptation. The same is true 

 of moraine, excepting that I think I have found some ancient moraines that have 

 been subsequently modified by the action of water, whereby the coarser detritus 

 has been more or less covered by water-worn and sorted materials. 



