Parr I. Secr.i.§2.] VOLCANIC DUST. . 219 
_ tion. But it has been observed at Santorin that though this is true 
in dry weather, the fall of rain with the dust at once acts detri- 
mentally. On the 8rd of June, 1866, the vines were there withered 
up as if they had been burnt along the track of the smoke cloud. 
By the gradual accumulation of volcanic ashes new. geological 
formations arise which, in their component materials, not only bear 
» witness to the volcanic eruptions which produced them, but pre- 
serve a record of the land-surfaces over which they spread.. In 
the third place, besides the distance to which the fragments may 
be hurled by volcanic explosions, or to which they may be diffused 
by the ordinary aerial movements, we have to take into account the 
vast spaces across which the finer dust is sometimes borne by upper 
air-currents. In the instance already cited ashes fromm Coseguina 
fell 700 miles away, having been carried all that long distance by a 
high counter-current of air, moving apparently at the rate of about 
7 miles an hour in an opposite direction to that of the wind which 
blew at the surface. By the Sumbawa eruption, also referred to 
above, the sea. west of Sumatra was covered with a layer of ashes two 
feet thick. On several occasions ashes from one of the Icelandic 
volcanoes have fallen so thickly between the Orkney and Shetland 
Islands, that vessels passing there have had the unwonted deposit 
shovelled off their decks in the morning. In the year 1783, during an 
eruption .of Skaptar-Jékull, so vast an amount of fine dust was 
ejected that the atmosphere over Iceland continued loaded with it 
for months afterwards. It fell in such quantity over parts of 
Caithness—a distance of 600 miles—as to destroy the crops; that 
year is still spoken of by the inhabitants as the year of “ the ashie.” 
Traces of the same deposit have been observed in Norway, and even 
as far as Holland. Hence it is evident that volcanic accumulations 
may take place in regions many hundreds of miles distant from any 
active voleano, A single thin layer of volcanic detritus in a group 
of sedimentary-strata would thus not of itself prove the existence of 
contemporaneous volcanic action in its neighbourhood. Unsup- 
ported by other proof of adjacent volcanic activity, it might be held 
to have been wind-borne from a volcano in a distant region. 
- Lava Streams.—At its exit from the side of a volcano, lava 
glows with a white heat, and flows with a motion which has been 
compared to that of honey or of melted iron. It soon becomes red, 
and, like a coal fallen from a hot fireplace, rapidly grows dull as it 
moves along, until it assumes a black, cindery aspect. At the same 
time the surface congeals, and soon becomes solid enough to support 
a heavy block of stone. The aspect of the stream varies with the 
composition and fluidity of the lava, form of the ground, angle of 
slope, and rapidity of flow. Viscous lavas, like those of Vesuvius, 
break upon the surface into rough brown or black cinder-like slags, 
and irregular ragged cakes, which, with the onward motion, grind 
and grate against each other with a harsh metallic sound, sometimes 
' Fouqué, op. cit. p. 81. 
