588 



NA TURE 



[_April 17, 1884 



a prism, of the brachypinacoid, and indications of the faces of a 

 pyramid. This augite is pleochroic and has a greenish tint, and 

 extinguishes in certain cases obliquely to the prismatic edges. It 

 is this character which often permits it to be distinguished from 

 rhombic pyroxene with which the augite is associated. The 

 crystals of hypersthene are transparent, of a deep brown colour, 

 strongly dichroic, with green and brown tints. They are in 

 rectangular prisms terminated by a pyramid, and extinguish 

 between crossed nicols parallel to their longitudinal edges. 

 Magnetic iron, which is rather abundant in the ashes, is recog- 

 nised in the form of grains and octahedrons. We have not been 

 able to detect with certainty either hornblende or olivine. The 

 largest grains of this ash are true microscopic lapilli, where we 

 distinguish in a vitreous mass microlithic crystals of felspar, of 

 magnetite, and more rarely of pyroxene. Finally, we observe 

 with the microscope particles of an organic origin, which are 

 easily recognisable by their fibrous and reticulated structure. 

 These impurities may have been transported by winds, or may 

 have come from the ground where the ashes were collected. ■ 



In spite of all the uncertainties which the exact diagnoses of 

 volcanic dust present, we can consider them often, from the 

 point of view of their mineralogical composition, as analogous 

 with the augite-andesites. We know, besides, that it is to these 

 rocks that the lavas of the volcano of Krakatoa should be 

 referred. 



The ashes which fell at Batavia on August 27, 1883, and 

 samples of which were sent to Holland by M. Wolf, resident on 

 that island, have been analysed with the following results : — 



I. I'lig grm. of substance dried at 110° C, and fused with 

 carbonate of soda and potash, gave 07799 grm. of silica, o'i754 

 grm. of alumina, o'o9li grm. of peroxide of iron, o'040i grm. 

 of lime, o'398 grm. of pyrophosphate of magnesia, answering to 

 O'oi434 grm. of magnesia. A recent determination of titanic 

 acid has given o'62 per cent. TiOo. 



II. I "222 grm. of substance dried at 110° C. gave o'0335grm. 

 of loss on ignition (water, organic substances, chloride of 

 sodium) ; the same substance treated with hydrofluoric and sul- 

 phuric acids gave o'ii6i grm. of chloride of sodium and potas- 

 sium, and o'oii8 grm. of chloroplatinate of potassium, answering 

 to o'oilS grm. of potash and to o'OiSS grm. of chloride of 

 potassium ; by difference = o'0973 grm. of chloride of sodium, 

 answering to o'osi63 of soda. 



III. 17287 grm. of substance dried at 110° C. was treated in 

 a closed tube with hydrofluoric and sulphuric acid. The oxida- 

 tion required 2 "3 cc. of permanganate of potash (l cc. = 0'02I2 

 grm. FeO), answering to o"047876 grm. of pero.xide;,of iron. 



65-04 

 1 4 '63 

 4'47 

 2-82 

 traces 

 I 20 

 J '34 

 0-97 

 4-23 

 274 



0-97 

 4-23 

 274 



99 '44 



It will be understood that it is barely possible to suljmit this 

 analysis to discussion. The abundance of vitreous particles in 

 the ashes renders illusory the calculation of the values obtained, 

 and the distribution of the substances among the different species 

 of constituent minerals. This vitreous matter can indeed con- 

 tain an indeterminate quantity of the different bases. On the 

 other hand, the difficulties of the calculation are all the greater, 

 as the constituent minerals of the ashes may contain, as iso- 

 morphs, the bases which the analysis suggests. It is none the 

 less true, however, that the percentage composition expressed by 

 the analysis supports the preceding mineralogical determinations, 

 without permitting the species to be precisely determined. It 

 agrees with the interpretation that the magma from which the 

 ashes were formed belongs to the augite-andesites. 



The vitreous and mineral fragments we have just described 

 rom the Krakatoa eruption being identical with those which w-e 

 encounter in deep-sea sediments, we may conclude that both 

 have a similar origin. In certain cases, however, we have in 

 place of augite a predominance of hornblende, and sometimes 

 black mica is abundant. Again, we find more or less fragment- 



ary crystals of peridote, of magnetite, of sanidine, and, more 

 rarely, of leucite and of hauyne. We can easily understand 

 this variation in composition, following the nature of the magma 

 from which the ashes collected in different regions of the sea 

 were derived. But in all cases it is the predominance of vitreous 

 particles, with their special structure, which indicates most 

 clearly the volcanic nature of the inorganic constituents of a 

 sediment. 



If now we consider the conditions which govern the distribu- 

 tion of ashes in the atmosphere or at the bottom of the sea, we 

 shall be able to show how it is that there is generally a pre- 

 dominance of vitreous particles in these ashes. In the first 

 place, these are vitreous matters rather than minerals, properly 

 so called, from the moment of ejection from the crater. More- 

 over we should, in a general way, not expect to find that inco- 

 herent eruptive matters, whicli are spread out at a distance from 

 the volcano, present a perfectly identical composition with those 

 other loose products, such as lapilli, volcanic bombs, and scoriae, 

 which are projected only a short distance from the focus of erup- 

 tion. Even where there exists a perfect chemical and mineralo- 

 gical identity, in the crater itself, between the lavas and the 

 pulverulent materials of the same eruption (the supposition being 

 that the ashes arise simply from the trituration of the lavas), we 

 can easily understand that these latter, being carried far and 

 wide by the winds, must undergo a true sorting in their passage 

 through the atmosphere, according to the specific gravity of the 

 amorphous elements or crystalline constituents. It residts from 

 this that, according to the points where they are collected, vol- 

 canic ashes may, although belonging to the same eruption, pre- 

 sent differences not only with respect to the size of the grains, 

 but also with respect to the minerals. 



In this mode of transport it is evident that the vitreous par- 

 ticles, other things being equal, will be transported farthest from 

 the centre. In the lirst place, they are more abundant tlian the 

 other particles, and again they possess in their chemical nature 

 and in their structure conditions which permit the aerial currents 

 to take them up and carry them to great distances ; they consist 

 of a silicate in which the heavy bases are poorly represented as 

 compared with the other constituent elements ; they are filled 

 with gaseous bubbles which lower their specific gravity, and at 

 the same time are capable of being broken up into the minutest 

 particles. The minerals with which they are associated at the 

 moment of ejection from the crater are not, like them, filled 

 with gaseous bubbles ; they do not break up so easily into im- 

 palpable powder, for they are not porous, and are not in the 

 same state of tension as the rajjidly-cooled vitreous dust. Finally, 

 many of these species are precisely those whose specific gravity 

 is very high, on account of the bases entering into their compo- 

 sition. These minerals will not then be carried so far from the 

 centre of eruption, and in all cases the vitreous particles are the 

 essential one's in the atmospheric dusts derived from volcanic 

 ashes. 



We have a beautiful illustration of this in the ashes of Kraka- 

 toa. In proportion as the ashes are collected at a greater dis- 

 tance from a volcano, so are they less rich in minerals, and the 

 quantity of vitreous matter predominates. According to a 

 verbal communication from Prof. Judd, the ashes collected at 

 Japan contain only a relatively small proportion of pyroxene and 

 magnetite. 



I f we wish to assure ourselves of the nature of an atmospheric 

 dust, and, as has lately been frequently attempted in Europe, to 

 show that the dust is really from the Krakatoa eruption, it is 

 important above all to seek for the presence of vitreous fragments. 

 Tire characters which we have indicated permit any one to 

 recognise them easily under the microscope. We would remark, 

 however, that the presence of crystals, either of hypersthene, ol 

 augite, or of particles of magnetite in an atmospheric dust col- 

 lected in Europe, does not prove in a certain manner that the 

 dust belongs to the ashes from Krakatoa ; for, besides the diffi- 

 culties of an exact mineralogical determination of the fragment- 

 ary elements, it is difficult to understand how these heavy mine- 

 rals should have been carried by the aerial currents, while the 

 vitreous dust is absent. As we have just shown, it is the con- 

 trary which should have taken place. 



It results as a corollary from these considerations that the 

 chemical composition of an ash may vary according to tlie point 

 at which it has been collected, and it tends also, other things 

 being equal, to become more acid the further it is removed from 

 the centre of eruption. If we admit, for example, that the 

 magma which gave birth to the ashes of Krakatoa is an augite- 

 andesite, as everything seems to indicate, the percentage of 



