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322 - DYNAMICAL GEOLOGY. [Boox TIT. 
On the sites of ancient monuments and cities this reproductive 
action of the atmosphere can be most impressively seen and most 
easily measured. In Europe on sites still inhabited by an abundant 
population, the deep accumulations beneath which ancient ruins 
often lie, are doubtless mainly to be assigned to the successive 
destructions and rebuildings of generation after generation of 
occupants. But at Nineveh, Babylon, and many other eastern sites, 
mounds which have been practically untouched by man for many 
centuries consist of fine dust and sand gradually drifted by the wind 
round and over abandoned cities, and protected and augmented by 
the growth of vegetation.’ In these arid lands the air is often laden 
with fine detritus, which drifts like snow round conspicuous objects 
and tends to bury them up in a dust-drift. In Central Asia, even 
when there is no wind, the air is often thick with fine dust, and a 
yellow sediment settles from it over everything. In Khotan an ex- — 
ceedingly fine dust sometimes so obscures the sun, that even at 
midday one cannot read large print without a lamp. This dust 
deposited on the soil heightens and fertilizes it, and is regarded by 
the inhabitants as a kind of manure, without which the ground would 
be barren.” 
Loess.—In the course of long ages, the constant deposit of dust 
has in these Asiatic countries formed a massive accumulation which — 
sweeps over the plateaux and rises to 6000 feet or more above the 
sea, and for wide spaces conceals all older formations. Richthofen 
describes it in China under the name of Loess, as a wholly unstratified 
formation of a yellowish calcareous clay, amounting sometimes to 
1500 or possibly over 2000 feet in thickness, having a tendency to 
split by vertical joints, and to form, along valleys and ravines, ranges 
of precipitous cliffs sometimes 500 feet high. It is firm enough to 
be excavated into tiers of chambers and passages by a teeming 
population. It contains abundant remains of land-shells, bones of 
land animals, and relics of a terrestrial vegetation. Richthofen 
distinguishes between the Jland-loess here described and lake-loess, 
where water has co-operated. : 
For atmospheric accumulations of this nature Trautschold has pro- 
posed the name eluvium. ‘They originate in situ, or at least only by 
wind-drift, whereas alluvium requires the operation of water, and 
consists of materials brought trom a greater or less distance.* For 
wind-formed deposits the term “olian” is sometimes used. 
Sand-hills or Dunes.—Winds blowing continuously upon sand 
drive it onward, and pile it into irregular heaps and ridges, called 
' The rubbish which in the course of many centuries has accumulated above the 
foundations of the Assyrian buildings at Kouyunjik was found by Layard to be in some 
places twenty feet deep. It consisted partly of ruins, but mostly of fine sand and dust 
blown from off the plains and mixed with decayed vegetable matter. Layard, Nineveh 
and its Remains, 3rd edit. ii. p. 120. See also Richthofen’s China, i. p. 97. 
? Johnson’s “ Journey to Hohi, thecapital of Khotan,” Journ, Geog. Soc. xxxvii. 1867, p. 1. 
® Richthofen’s China, i. cap. ii. T. W. Kingsmill (Q. J. Geol. Soc. xxyii. p. 376) 
advances the untenable theory that this loess is of marine origin. 
* Z, Deutsch, Geol, Ges. xxxi. p, 578, 
