lcanic dust being carried enormous
distances through the atmosphere sufficiently prove the importance of
wind as a carrier of solid matter, but unfortunately the matter
collected has not been hitherto examined with a view to determine the
maximum size and weight of the particles. A few facts, however, have
been kindly furnished me by Professor Judd, F.R.S. Some dust which fell
at Genoa on 15th October 1885, and was believed to have been brought
from the African desert, consisted of quartz, hornblende, and other
minerals, and contained particles having a diameter of 1/500 inch, each
weighing 1/200,000 grain. This dust had probably travelled over 600
miles. In the dust from Krakatoa, which fell at Batavia, about 100 miles
distant, during the great eruption, there are many solid particles even
larger than those mentioned above. Some of this dust was given me by
Professor Judd, and I found in it several ovoid particles of a much
larger size, being 1/50 inch long, and 1/70 wide and deep. The dust from
the same eruption, which fell on board the ship _Arabella_, 970 miles
from the volcano, also contained solid particles 1/500 inch diameter.
Mr. John Murray of the _Challenger_ Expedition writes to me that he
finds in the deep sea deposits 500 and even 700 miles west of the coast
of Africa, rounded particles of quartz, having a diameter of 1/250 inch,
and similar particles are found at equally great distances from the
south-west coasts of Australia; and he considers these to be atmospheric
dust carried to that distance by the wind. Taking the sp. gr. of quartz
at 2.6, these particles would weigh about 1/25,000 grain each. These
interesting facts can, however, by no means be taken as indicating the
extreme limits of the power of wind in carrying solid particles. During
the Krakatoa eruption no gale of special violence occurred, and the
region is one of comparative calms. The grains of quartz found by Mr.
Murray more nearly indicate the limit, but the very small portions of
matter brought up by the dredge, as compared with the enormous areas of
sea-bottom, over which the atmospheric dust must have been scattered,
render it in the highest degree improbable that the maximum limit either
of size of particles, or of distance from land has been reached.

Let us, however, assume that the quartz grains, found by Mr. Murray in
the deep-sea ooze 700 miles from land, give us the extreme limit of the
power of the atmosphere as a carrier of so