574 GEOLOGICAL IMPORTANCE OF DUNES. 
Tf the sand of coast dunes is, as Staring describes it, com- 
posed chiefly of well-rounded quartzose grains, fragments of 
shells, and other constant ingredients, it would often be recog- 
nizable as coast sand, in its agglutinate state of sandstone. The 
texture of this rock varies from an almost imperceptible fine- 
ness of grain to great coarseness, and affords good facilities for 
microscopic observation of its structure. There are sandstones, 
such, for example, as are used for grindstones, where the grit, 
as it is called, is of exceeding sharpness; others where the 
angles of the grains are so obtuse that they scarcely act at all 
on hard metals. The former may be composed of grains of 
rock, disintegrated indeed, and re-cemented together, but not, in 
the meanwhile, much rolled ; the latter, of sands long washed 
by the sea, and drifted by land-winds. There is, indeed, so 
much resemblance between the effects of driving winds and of 
rolling water upon light bodies, that there might be difliculty 
in distinguishing them; but after all, it is not probable that 
sandstone, composed of grains thrown up from the salt sea, and 
long tossed by the winds, would be identical in its structure 
with that formed from fragments of rock crushed by mechani- 
cal force, or disintegrated by heat, and again agglutinated with- 
out much exposure to the action of moving water. 
and probably the presence of certain salts, have cemented so as to form true 
sandstone, soft indeed, but which does not yield except to the pickaxe. These 
sandstones exhibit an inclination which seems to be the effect of wind; for 
they conform to the direction of the sands which roll down a scarp occasioned 
by the primitive obstacle.” 
‘“ At New Quay the dune sands are converted to stone by an oxide of iron held 
in solution by the water which pervades them. This stone, which is formed, so 
to speak, under our eye, has been found solid enough to be employed for build- 
ing.”—Esquiros, L’ Angleterre, etc., in Revue des Deux Mondes, 1864, pp. 
44, 45. 
The dunes near the mouth of the Nile, the lower sands of which have been 
cemented together by the infiltration of Nile water, would probably show a 
similar stratification in the sandstone which now forms their base. 
Dana describes a laminated rock often formed by the infiltration of water 
into the sand dunes on the Hawaian islands.— Corals and Coral Islands, 1872, 
p- 155, 
