1870.] 225 [Shaler. 



undulations, the summits of which are generally parallel to the shore. 

 On the innermost of the islands the action of the weather has partly 

 obliterated these reliefs, but over a large part of the territory they are 

 still quite conspicuous. 1 



On St. Helena Island they are peculiarly distinct, for the valleys 

 between the summits of the ridges, though they are only a few feet 

 deep, are still depressed enough to convert their bases into swamps, 

 so that the alternation of upland and morass in parallel lines charae- • 

 terizes a large part of the surface of this and the adjoining islands. 

 It is clear, on even a casual inspection, that these reliefs are not the 

 product of aerial erosion; their channels are rarely occupied by 

 streams ; indeed, one may travel for days among these islands without 

 seeing any indication of subaerial erosion, except from tidal currents 

 wearing away some low cliff. There can be no doubt that this con- 

 tour of surface is due to submarine forces, and that the essential 

 features of the topography of this region were impressed upon it 

 before it came out of the sea. Something of this same character of 

 surface may be found beneath the level of the ocean along this coast, 

 though it is at no point so clearly traceable as on the surface of the 

 islands. There can be little doubt that these ridges and furrows are 

 due to the run of tidal currents along the shore. There seems to be 

 a tendency in streams not bounded by resisting banks, such as the 

 tidal streams which course along a shallow shore, to arrange the ma- 

 terial they sweep over in long ridges. Such a stream does not always 

 press equally upon its floor, but is apt to have a banded character, or 

 to have a form which may be compared to several streams flowing 

 side by side, and closely joined with each other. Just what this is 

 owing to it is not easy to say, but it seems not altogether improbable 

 that the peculiar alternate strips of hot and cold water noticed in 

 the Gulf Stream by the officers of the Coast Survey, may be due to 

 the same or a related cause. The action of currents of air upon in- 

 coherent vapor in the atmosphere forming the banded clouds called 

 by sailors mares' tails, may possibly be due to the same tendency. 



In order to understand just how the sea acted upon this surface as 

 it began to be lifted above it, it must be noticed that although the 

 tides at Cape Hatteras or Cape Florida are not more than two feet in 



1 1 am much indebted to Capt. C. O. Boutelle, of the U. S. Coast Survey, for 

 information on many points connected with the topography of this region, both 

 subaurial and submarine, and especially for having called my attention to these 

 parallel ridges on Hilton Head Island. .. fl 



PROCEEDINGS B. S. N. H.— VOL. XIII. 15 APRIL, 186&. 



