= 5 7 * Pa en's “ os Mor eS ae 
x - ES 7 ae - cas 
~ df = ft 
396 =«~-~S—s- DYNAMICAL GEOLOGY. [Boox TIT. 
land. Rain falling through such a dust-cloud mixes with it, and 
descends either on sea or land as what is popularly called “blood- 
rain.” This is frequent on the north-west of Africa, about the Cape 
Verd Islands, in the Mediterranean, and over the bordering countries. 
A microscopic examination of this dust by Ehrenberg led him to the 
belief that it contains numerous diatoms of South American species ; 
and he inferred that a dust-cloud must be swimming in the 
atmosphere, carried forward by continuous currents of air in the 
region of the trade-winds and anti-trades, but suffering partial and 
periodical deviations. But much of the dust seems to come from the 

sandy plains and desiccated pools of the north of Africa. Daubrée — 
recognized in 1865 some of the Sahara sand which fell in the Canary 
Islands. On the coast of Italy a film of sandy clay, identical with 
that from parts of the Libyan desert, is occasionally found on windows 
after rain. In the middle of last century an area of Northern Italy, 
estimated at about 200 square leagues, was covered with a layer of 
dust which in some places reached a depth of one inch. In 1846 
the Sahara dust reached as far as.Lyons. Should the travelling 
dust encounter a cooler temperature, it may be brought to the ground 
by snow, as has happened in the north of Italy, and more notably 
in the east and south-east of Russia, where the snows are some- 
times rendered dirty by the dust raised by winds on the Caspian 
steppes. It is easy to see that a prolonged continuance of this 
action must give rise to widespread deposits of dust, mingled with 
the soil of the land, and with the silt and sand of lakes, rivers, and 
the sea; and that the minuter organisms of tropical regions may 
thus come to be preserved in the same formations with the terrestrial 
or marine organisms of temperate latitudes.* 
The transport of volcanic dust by wind, already (p. 219) referred 
to, may be again cited here as another example of the geological work 
of the atmosphere. ‘Thus from the Icelandic eruptions of 1874-75 vast 
showers of fine ashes not only fell on Iceland to a depth of six inches, 
destroying the pastures, but were borne over the sea and across 
Scandinavia to the east coast of Sweden. Considerable deposits of 
volcanic material may thus in the course of time be formed even far 
remote from any active volcano. ? 
Transportation of Seeds.—Besides the transport of dust 
and minute organisms for distances of many thousands of miles, wind 
may also transport living seeds, which, finally reaching a congenial 
climate and soil, may take root and spread. We are yet, however, 
very ignorant as to the extent to which this cause has actually operated 
in the establishment of any given local flora. With regard to the 
minute forms of vegetable life, indeed, there can be no doubt as to 
the efficacy of the wind to transport them across vast distances on 
1 See Humboldt on dust whirlwinds of Orinoco, “ Aspects of Nature;” also Maury, 
“Phys. Geog. of Sea,” chap. vi.; Ehrenberg’s “ Passat-Staub und Blut-Regen,’’ Berlin 
Akad, 1847. A paper by A. von Lasaulx on so-called cosmic dust hag just appeared in 
T'schermak’s Mineral. Mittheil. 1880, p. 517. 
* Nordenskidld, Geol. Mag. (2), ili. p. 292, 
