Trans. N. Y. Ac. Set. 64 Feb. i8, 



tual attrition. During their short period of aerial 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, i.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, o.o I — 0.09 Less than 0.03 



B. Fine red sand of the Sahara 0.075 — o-30 0.075 — o. 125 



C. Coarse dust from vessel's deck 0.20 — 0.74 0.25 — 0.36 



D. Fine dust from vessel's deck 0.07 — 0.35 o.io — 0.18 



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 



