VratsoN. Ys AeNSc. 64 Feb. 18, 
tual attrition. During their short period of aérial transport, there 
seems to have been little contact between the particles of this ash. It 
appears wholly made up of a colorless obsidian, showing the usual op- 
tical behavior of a glass, with the exception of occasional pale lines of 
refraction. These are due to an almost universal fibrous structure, 
or penetration of an amorphous glassy groundmass by fine parallel 
lines. The fracture has been strongly influenced by this structure, 
so that many particles have linear forms, which, in association with 
their fibrous structure, make them resemble in ordinary light the 
grains of some mineral with strong cleavage. About one-third of the 
ash consists of minute particles, not exceeding a few thousandths of a 
millimetre in diameter. The measurements of the diameters of the 
particles of the greater portion, z.e., about two-thirds of this ash, are 
given (A) in the table below, in fractions of a millimetre. For the sake 
of comparison, there are also given (B) the measurements of the fine 
reddish angular sand of the Sahara, consisting almost entirely of glassy, 
milky, and reddish quartz, and a little chalcedony and chert, and (C 
and D) of two samples of a fine dust, evidently derived from the Sa- 
hara, which fell a half century ago upon the deck of a vessel, fifty miles 
off the coast of Africa. The sample C consists of round grains, largely 
cellular, composed of glassy and milky quartz and of chert, chalcedony, 
and foraminifera. The sample D was mostly made up of angular par- 
ticles of glassy and milky quartz, chert, and chalcedony, intermixed 
with coarser rounded grains of the same. 
Range. Average. 
A. Gray ash of Krakatoa, greater portion. 0.01 —o.09 Less than 0.03 
iB. fine-red sand.ot the Sahara ye; on. a. 0.07 5—0.30 0.075—0.125 
G: Coarse dust trom vessel's deck... ..... 0.20 —0.74 0.25 —0.36 
D> Fine dust from vessel's deek. .-0... . 0.07 —0.35 0.10 —o.18 
It will be seen that the particles of the Sahara sand are, on an aver- 
age, from two to four times as large as those of the ash of Krakatoa, 
but that those of the dust C are from eight to twelve times as large, 
and those of the dust D, from three to six times as large. As the 
specific gravity of obsidian (2.3 to 2.5) is decidedly less than that of 
quartz (2.5 to 2.8), it would not be astonishing that the exceedingly 
fine particles of the ash should have suffered a vastly wider transport. 
The PRESIDENT observed that he had recently heard much said on 
the subject of the origin of the ‘‘ red skies,” in opposition to the sup- 
posed absurdity of so wide a transport of the ash of Krakatoa, but 
there was no other known source of such pulverulent matter, and 
there could be no doubt that the volcanic ashes might be transported 
any distance. The known system of atmospheric circulation would 
