268 THE PLANT COVERING OF OCRACOKE ISLAND. 



have been made upon these islands.^ Kerr further states that * ' the reef 

 is increasing in continuity and breadth." But this is not the gen- 

 eral opinion, for it is said that there is to-daj' water of considerable 

 depth where houses stood upon Ocracoke within the memory of living 

 men, and it is stated^ that "a fine fig orchard and many peach trees, 

 with a fine potato patch and garden," occupied earlier what is now 

 Hatteras Inlet. That the present tendency of this whole coast line is 

 one of subsidence can hardly be disputed. 



Beneath the superficial Recent deposits of dune sands and salt- 

 marsh silt which cover the greater part of the island lie the sands 

 and clays of the Columbia formation, which extend to a considerable 

 but unascertained depth. This and the Recent accumulations are 

 the onh^ geological formations of this part of the coastal plain which 

 need be considered in relation to the existing plant covering. 



Excepting the areas occupied by creeks and salt marsh, the soil of 

 Ocracoke is a fine white marine sand, almost everywhere devoid of 

 any considerable admixture of humus. Only in the live-oak groves 

 is there enough A^egetable matter present to give the sand a gray 

 color. There is doubtless some quantity of calcium carbonate in the 

 soil, owing to the presence of small particles of shells washed up by 

 the waves and scattered b}^ the wind.-^ 



As much of the island is subject to occasional inundation and to 

 the deposition of spray by the winds, the soil content of sodium chlo- 

 rid must be considerably greater at times than in ordinary inland soils.^ 

 There is no lack of moisture in this sandy substratum. Even in the 

 driest looking beach sand, water usually stands at a depth of only 15 

 to 30 centimeters (6 to 12 inches) from the surface. The superficial 

 layer of the sand acquires a great amount of heat on sunny days and 

 becomes thoroughly desiccated, in which condition it is subject to 

 being blown about by the wind, its degree of coherenc}^ depending 

 upon the character of the vegetation. At night, however, sand gives 

 up its heat rapidly and absorbs much dew, if conditions are favorable.'^ 



The soil of the salt marsh, which appears to be usually a thin sheet 



' The succession of strata in the North Carolina coastal plain, where exposed 

 in the valleys of the Neuse and Cape Fear rivers, is given as follows, beginning 

 with the oldest: 



(1) Potomac gravel, sands, and clays. 



(2) Cretaceous sands and clays. 



(3) Tertiary (Eocene and Miocene) marls and clays. 



(4) Lafayette (yellowish and brownish sands and loams). 



(5) Columbia sands, gravels, and clays. 



2W. L. Welch, Bui. Essex Inst., vol. 17, pp. 87 to 42. 1886. 



■^According to Contejean (Geogr. Bot.),the proportion of calcium carbonate 

 thus supplied to the strand soils is insignificant except near the wave limit, the 

 particles being soon dissolved by the carbon dioxide contained in rain water and 

 then washed down through the readily permeable soil. 



^Sea water contains from 2.7 to 3.2 per cent of NaCl. 



^Warming, Lehrbuch, p. 66. 



