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320 DYNAMICAL GEOLOGY. [Boor Til. 
heated up to 187° Fahr., cooled so rapidly by radiation at night that, 
unable to sustain the strain of contraction, they split and threw off 
sharp angular fragments from a few ounces to 100 or 200 Ib. 
in weight.! In the plateau region of North America, though the 
climate is too dry to afford much scope for the operation of frost, 
this daily vicissitude of temperature produces results that quite 
rival those usually associated with the work of frost. Cliffs are 
slowly disintegrated, the surface of arid plains is loosened, and the 
fine débris is blown away by the wind. 
Effects of wind.—tThe geological work directly due to the 
air itself is mainly performed by wind. A dried surface of rock or 
soil, when exposed to wind, has the finer disintegrated particles blown 
away as dust or sand. ‘This process, which takes place familiarly 
before our eyes on every street and roadway, may be instructively 
observed over cultivated ground, as well as on tracts with which 
man has not interfered. It is most marked in arid climates. Many 
old fortifications in Northern China, for example, have been laid 
bare to the very foundations by the removal of the surrounding soil 
through long-continued action of wind.” In the dry plateaux of 
North America, too, though no human memorials serve there as 
measures, extensive denudation from the same cause is in progress. 
Not merely does the wind blow away what has already been 
loosened and pulverized. The grains of dust and sand are them- 
selves employed to rub down the surfaces over which they are 
driven. ‘The nature and potency of the erosion done by sand grains 
in rapid motion is well illustrated by the artificial sand-blast, in 
which a spray of fine siliceous sand driven with great velocity is 
made to etch or engrave glass. ‘The abrading and polishing effects 
of wind-blown sand have long been noticed on Lgyptian monuments 
exposed to sand-drift from the Libyan desert. Similar effects have 
been observed on dry volcanic plains of barren sand and ashes, as on 
the island of Volcano? On the sandy plains of Wyoming, Utah, and 
the adjacent Territories, surfaces even of such hard materials as 
calcedony are etched into furrows and wrinkles, acquiring at the 
same time a peculiar and characteristic polish. There, also, large 
blocks of sandstone or limestone which have fallen from an adjacent 
cliff are attacked, chiefly at their base, by the stratum of drifting 
sand, until by degrees they seem to stand on narrow pedestals. 
As these supports are reduced in diameter the blocks eventually 
tumble over, and a new basal erosion leads to a renewal of the same 
stages of waste. Hollows on rock surfaces may also be noticed 
where grains of sand, or small pebbles kept in gyration by the wind, 
gradually erode the cavities in which they lie. 
As the result of the protracted action of wind upon an area 
' Livingstone’s Zambesi, pp. 492, 516, 
* Richthofen’s China, Berlin, 1877, i. p. 97. 
® Kayser, Z, Deutsch. Geol. Ges. xxvii. p. 966. 
* See Gilbert in Wheeler’s Report of U.S: Geograph. Surv. W. of 100th Meridian, iii. 
p. 82, Blake, Union Pacific Railroad Report, y. pp. 92, 230, 
