CHAP. IV.] ST. THOMAS TO BERMUDAS. 287 
by the engineers for military works. The softer limestones 
are more frequently used for ordinary buildings. The stone 
is cut out of the quarry in rectangular blocks by means of a 
peculiarly constructed saw, and the blocks, at first soft, harden 
rapidly, like some of the white limestones of the Paris basin, 
on being exposed to the air. 
As I have already indicated, this limestone is entirely what 
General Nelson aptly calls an “A¢olian formation.” The fine 
coral-sand, which surrounds the islands to a distance of about 
twenty miles, is washed in by the sea; it is then caught at 
certain exposed points by the prevailing winds and blown 
into sand-hills, often forty or fifty feet in height. The sana 
is spread over the surface in a certain sense uniformly, but that 
uniformity is liable to be interfered with by any thing which 
for a moment affects the direction or force of the wind; for 
instance, the sand is blown up and heaped round any obstacle, 
or it may be swept out by irregular gusts into hollows which 
are afterward filled up by a secondary series of layers, or a 
total change may be made on the whole arrangement of the 
surface by-a subtropical rain-flood. All the appearances pro- 
duced with great rapidity by such causes are, of course, per- 
petuated in the rock which is formed by the consolidation of 
the sand, so that we have often repeated again and again in the 
distance of a quarter of a mile all the phenomena—denudation, 
unconformability, curving, folding, synclinal and anticlinal 
axes, ete.—which are produced in real rocks, if I may use the 
expression, by combined aqueous and metamorphic action, 
extending over incalculable periods of time. The principal 
roads, which are extremely good, as they are laid out and main- 
tained partly with a view to military operations, frequently 
pass through deep cuttings, and give excellent geological sec- 
tions, exhibiting an amount of confusion sufficient to perplex 
the most experienced geologist, if he did not hold the key. 
The general tendency of the layers of sand to wrap themselves 
