THE GEOLOGY OF BERMUDA. 15 
islands, by making quite deep cuts wherever they 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 LV. 
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 me 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 lessenéd. 
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 Elisée Reclus. New York, 1873. p. 195. 
