ROCKS 59 
acts as a kind of sand-blast which abrades, frets, honeycombs, 
and undermines them—in other words, the sand that results 
from the action of insolation grinds and reduces to sand and 
dust the exposed rock-surfaces against which it is borne. As 
the travelling sand-grains, which seldom rise more than a 
few feet above the surface, are continually subject to mutual 
attrition, both in the air and upon the ground, they tend to 
become more or less well rounded. This character often 
serves to distinguish desert blown sand from sand of alluvial 
origin—the smaller grains of which are rather angular or 
subangular in shape. Having been carried mainly in suspen- 
sion, they escape the constant trituration to which the grains 
of blown sand are subject. Blown sand, as a rule, consists 
principally of quartz—the commonest and one of the hardest 
of rock-forming minerals. In coastal tracts, however, the 
dunes, while consisting chiefly of quartz, often contain many 
other ingredients, more especially comminuted shell-débris. 
Further, the grains of coastal blown sand are not infrequently 
coarser and less well rounded than those of desert sand. 
Dust is pre-eminently a product of relatively dry regions 
and of deserts—wherever, indeed, the land is naked or only 
partially clothed with vegetation, dust is formed, and may be 
‘swept up and transported by the wind. While the blown 
sand of a desert rises only a few feet or yards above the 
ground, the powdery dust is often swept upwards to a great 
height, and may be transported for hundreds or thousands of 
miles from the place of its origin. The fine-grained, homo- 
geneous, calcareous, and sandy loam known as Loess, which 
occupies wide areas in middle and south-eastern Europe, and 
covers vast tracts in China, is supposed by many geologists 
to be essentially a dust deposit or “steppe formation.” 
2, SEDIMENTARY ROCKS 
The rocks included under this head owe their origin to the 
mechanical action of water, and are usually, therefore, 
arranged in layers or beds. They vary exceedingly in texture 
.—from coarse aggregates of boulders and shingle to sedi- 
ments composed of the finest impalpable materials. Less 
sharply distinguished from each other, as a rule, than is the 
