THE GEOLOGY OF BERMUDA. 15 



islands, by making (juite deep cuts wherever tliey are required. These 

 cuttings are of great interest to the geologist, from the beautiful illus- 

 trations which they afford of that extreme irregularity of lamination 

 which is characteristic of wind-drifts. Not only the country roads, but 

 also the streets of the towns abound in these beautiful and instructive 

 sections. Fine exhibitions of this same structure are to be seen in the 

 natural sections afforded by the cliffs and pinnacles of the shore. The 

 characteristic structure of the drift-rock is shown in plates III and IV. 



The height of these accumulations of wind-blown sands is certainly 

 remarkable. The highest hills on the islands attain an altitude of about 

 250 feet; and, since no rock of marine formation has been observed at 

 an elevation of more than about 15 feet, it is evident that nearly the 

 whole elevation of these hills must be due to the accumulated sand- 

 drifts. Sand-drifts, however, of such extraordinary altitudes, though 

 exceptional, are by no means unparalleled. Prof. W. C. Kerr, State 

 Geologist of North Carolina, informs rae that sand-hills more than 100 

 feet in height occur along the coast of that State. Dunes of even greater 

 altitude than those in Bermuda occur on the coast of Gascony and near 

 Cape Verd.* 



In one respect, it seems to me, calcareous sands are better adapted 

 than silicious ones for the formation of hills of great height: viz., the 

 comparative solubility of the material, producing a more rapid consoli- 

 dation by the cementing of the grains. At times when the direction of 

 the wind is unfavorable to the increase of a sand-hill, in a region of 

 variable winds, the tendency will be to reduce the height by removal of 

 the sands from the summit. If the sand has already become partly con- 

 solidated, the loss from this cause will be much lessened. 



MOVEMENTS OF ELEVATION AND SUBSIDENCE. 



The facts which have been already detailed in regard to the distribu- 

 tion of the various kinds of rock, and other facts which will presently be 

 referred to, afford clear evidence as to changes of level which the islands 

 have undergone. The occurrence of beach-rock above the water-level, 

 as noticed at several localities, is of course unquestionable proof of ele- 

 vation. Proofs of subsidence are equally clear. The relation of the 

 beach-rock and drift-rock at Devonshire Bay and various other localities 

 along the south shore is evidence of subsidence. The cliff of drift-rock 

 which in these localities rises immediately back of the narrow platform 



* The Ocean, Atmosphere, and Life. By filisde Reclus. New York, 1873. p. 195. 



