586 



NA TURE 



\_Aprii 1 7, I < 



of our interpretations than the microscopic study of the ashes 

 from Krakatoa, whose mineralogical and chemical composition 

 M. Renard ' was the first to make known, and whose observa- 

 tions on this subject have been amply confirmed by the later 

 researches of other mineralogists. On the other hand, the con- 

 ditions under which floating pumice was found after that erup- 

 tion agree jjerfectly with the interpretation given eight years ago 

 by Mr. Murray relative to the mode of transport of these 

 vitreous matters and of the accumulation of their triturated 

 debris on the bottom of the ocean. We shall also see how the 

 sorting which takes place in the transport of the ashes of a vol- 

 cano has its analogy in what we find in the deep-sea deposits. 



In the first part of this communication we shall give the miner- 

 alogical description of the fragmentary products of Krakatoa, 

 and consider generally the observations relative to these aslies. 

 We shall also give the diagnostic characters of this volcanic dust, 

 and of all similar particles which we find in deep-sea deposits. 

 In the second part we will treat of the cosmic matters found in 

 the abysmal regions of the ocean, to which Mr. MuiTay was the 

 first to draw attention, and discuss their origin and distribution. 



First Part 



It is unnecessary to refer to the abundance of floating pumice, 

 to its various degrees of alteration, to its conveyance by means 

 of rivers, waves, and currents, and to its universal presence in 

 deep-sea deposits, which have been pointed out in some detail 

 in Mr. Murray's paper above referred to ; but we will briefly 

 recapitulate the characters of these volcanic matters, in accord- 

 ance with the examination we have made of a large number of 

 soundings and dredgings. We need not describe in detail the 

 special characters of the lapilli which have been brought up in 

 the dredge and sounding-rod from great depths. These frag- 

 ments of more or less scoriaceous rocks belong to the same litho- 

 logical varieties as those derived from terrestrial volcanoes. 

 They consist of fragments of trachyte of various dimensions, of 

 basalt, and, above all, of augite-andesite ; the most remarkable, 

 beyond all question, being lapilli of sideromelan, which are 

 often entirely transformed into palagonite, and pass into the clay 

 which is found so widely distributed, especially in the Pacific. 



We do not projjose here to take up in detail the wide distribu- 

 tion of the materials ejected from Krakatoa ; we are engaged in 

 collecting these, and will place the observations on maps along 

 with those of Mr. Buchan on the upper currents of the atmo- 

 sphere, which will be published in the Challengtr Reports. 



Before, liowever, passing to the description of the ashes them- 

 selves, we will briefly refer to some points touched upon by Mr. 

 Murray in his paper. It is there pointed out that, in regions far 

 removed from coasts, rounded fraginents of pumice were col- 

 lected on the surface of the sea by means of the tow-net, and 

 that, at certain points on the bottom of the ocean, the greater 

 part of the deposit is composed of vitreous splinters derived from 

 the trituration of pumice-stones. The description • of the pheno- 

 mena connected with the Krakatoa eruption gives us a complete 

 explanation of these observations. The specimens of pumice 

 from Krakatoa, which have been collected floating on the sea 

 and which we have examined, are in like manner rounded. The 

 angular surfaces are all worn away just as in pebbles ; the only 

 asperities to be observed consist of crystals and fragments of 

 crystals, which project beyond the general surface of the vitreous 

 matter, which last, on account of its structure, presents less re- 

 sistance to wear and tear than the minerals which are embedded 

 in it. 



We may recall the fact that the Bay of Lampong, in the 

 Straits of Sunda, was blocked by the vast accumulation of 

 pumice, formed in a few hours by the eiiiption of Krakatoa, 

 which completely filled the bay. This floating bar of pumice- 

 stones was about 30 km. long, i km. broad, and 3 m. to 4 m. in 

 depth, 2 m. or 3 m. of which were beldw the surface of the 

 water, and i m. above. These numbers give about 150 millions 

 of cubic metres of ejected matter. This mo_ving elastic wall rose 

 and fell with the waves and tide," and was carried by currents 

 thousands of miles from the point of eruption over the surface of 

 the ocean. The rounded form of blocks of pumice met with 

 everywhere floating on the surface of the sea, as well as of 

 those samples which, after having floated some time, became 

 waterlogged and sank to the bottom, may be perfectly explained 

 if we remember the friability of this rock, and, at the same time, 



' "Les centres volcaniques de IVruption du Krakatau " (Bull. Acad. 

 Roy. de Bclgique. scr. 3. t. vi. No. n, Stftnce du Nov 3, 1883). 



Cotnptes rendus di; VAcadnnic des Sciences, November 19, 1883. 



tile agitation to which it is submitted by the waves, through 

 which tlie pieces are continually being knocked against eacli 

 other. We understand also how this wear and tear gives rise tu 

 an immense quantity of pulvendent pumice fragments, which 

 contribute in a great measure to the formation of oceanic deposits. 

 .\s a matter of fact, rounded fragments of pumice have been met 

 with floating on the surface of every ocean, and during the last 

 few years many samples have been sent to us by captains of 

 ships and missionaries. As has been already pointed out, they 

 are universally distributed in oceanic deposits, although fre- 

 quently highly altered. 



I f it be easy to pronounce upon the volcanic nature of these 

 larger fragments, it becomes, on the other hand, exceedingly 

 difiicult when we have to deal with particles reduced to powder, 

 and when recourse must be had to the microscope. Let us see 

 what are the microscopic characters by which we recognise the 

 particles of this dust. 



We may here point out that it is not so much tlie presence oi 

 volcanic minerals which enables us in a marine sediment, as well 

 as in an atmospheric dust like the ashes of Krakatoa, to recog- 

 nise that the small fragments have an eruptive origin, as the 

 microscopic structure of the small vitreous particles. It is well 

 known that minerals reduced to small dimensions and irregularly 

 fractured, as in the case of volcanic ashes, often lose their dis- 

 tinctive characters. Their size does not allow us to judge of 

 their optical properties ; their form, irregular and fragmentary, 

 renders it difiicult to determine the characteristic extinction of 

 the species ; the phenomena of coloration, of pleochroism, and 

 the tint peculiar to the mineral, all lose so much of their intensity 

 that they no longer serve fortheidentificationof isolated minerals 

 like those of the volcanic ashes which we have to study. As a 

 result of our observations, we believe that in most cases where a 

 mineral, under the conditions we have just described, reaches 

 dimensions less than 0'05 mm., its determination with certainty 

 is no longer possible, and consequently its origin can no longer 

 be established ; whilst a vitreous fragment, like those of volcanic 

 ashes or triturated pumice, continues to be discernible when its 

 dimensions are less than o'oo5 mm. A reason for showing that 

 the absence or rarity of crystals, or of fragments of volcanic 

 crystals, ought not to be taken as a proof tliat a sedimentary 

 matter, either from the atmosphere or from the deep sea, is not 

 of volcanic origin, is the sorting process to which these matters 

 are subjected in the air and in the water, a phenomenon to 

 which we shall presently recur. 



The most reliable distinctive character is always found in the 

 structure of the small vitreous particles which are derived from 

 the trituration of pumice or have an analogous origin, inasmuch 

 as they have been ejected from the volcano in the state of ash. 

 The stnicture peculiar to these materials is seen in their fracture, 

 which leaves its impress upon the smallest fragments of debris, 

 in which the microscope can decipher no characteristic properties 

 except such as have relation to form. In order to assure our- 

 selves that these characters of pumice remain constant to the 

 extreme limits of pulverisation, such as are employed in the 

 preparation of silicates for chemical analysis, we pounded in an 

 agate mortar several varieties of pumice, and the powder thus 

 produced clearly showed itself to be composed of particles in 

 which were recognisable, with little trouble, the characters of 

 the pumice-like material which is constantly met with in the 

 sediments, and of which the ashes of Krakatoa give us beautiful 

 examples. The diagnostic character to which we here make 

 allusion rests on the distinctive peculiarities of incoherent vol- 

 canic products. What distinguishes them from lavas is not 

 merely the extraordinary abundance of vitreous matters, but also 

 the prodigious number of gas-bubbles which are inclosed by the 

 pumice and vitreous volcanic sands and ashes. These bubbles 

 are due to the expansion of the gases dissolved in the magma, 

 which also determine the eruption. If we admit, as everything 

 seems to show, that these incoherent volcanic matters are the 

 products of the pulverisation of a fluid magma, we can under- 

 stand that these particles, on cooling rapidly, will remain in the 

 vitreous state, and, on the other hand, that the dissolved gases, 

 yielding to the expansion, will form numerous pores which will 

 become elongated owing to the mode of projection. It is the 

 existence of these bubbles, or of such a filamentous structure, 

 which points out to us the vitreous volcanic materials in spite of 

 the great fineness of subdivision. It is also this structure which 

 allows these bodies to be carried to such great distances from the 

 scene of eruption. 



The examination of the Krakatoa ashes, and of the dust re- 

 sulting from the [lulverisation of the pumice of that volcano. 



