I 



/. 14. 1875] 



NA rURE 



52f 



That the silicates contained in meteorites should be identical, 

 or nearly so, with corresponding minerals in our globe seems 

 only the natural consequence of the identity in the elements 

 that constitute both. They are essentially magnesium silicates — 

 namely, olivine the basic, and enstatite (or bronzite) the neutral 

 silicate, the latter taking the form of augite to an amount corre- 

 sponding to the calcium present, where this latter element is a 

 constituent of the meteorite. Where, at the first production of 

 the meteoric minerals by the union of their elements, the oxygen 

 was in sufficient amount to allow of a portion of the iron pre- 

 sent being in the state of an oxide, ferrous oxide is combined in 

 the silicate, and the meteoric olivines are from this cause gene- 

 rally ferriferous, and the enstatite also assumes one of the varie- 

 ties of that mineral which the mineralogist has termed bronzite. 

 The silicic acid is rarely in excess of the amount requisite to 

 form an enstatite or augite ; usually the contrary condition is 

 evidenced by the presence of some olivine. The case of the 

 occurrence of free silica in the Brcitenbach meteorite, at present 

 exceptional, may, however, hereafter prove to be characteristic 

 of a type, and its occurrence, not as quartz, nor even as tridy- 

 mite (the crystallised silica discovered by von Rath), but in the 

 form to which I gave the name asmanite, in crystals belonging to 

 the orthorhombic system with the specific gravity of fused quartz, 

 seems to point to conditions, probably involving an enormous 

 temperature, as those under which such meteorites have been 

 formed, and such as have not been realised in the production of 

 any of the acid or super-siliceous silicates of our globe. The 

 felspathic ingredients of meteorites are for the most part basic, 

 chiefly consisting of anorthite, the most basic of terrestrial 

 felspars, known as a crystallised mineral in volcanic rocks. 

 Crystals of meteoric anorthite were measured by Viktor von Lang 

 at the British Museum, with results quite concordant with those 

 yielded by the crystals from the volcanoes of our planet. A 

 felspar with a composition corresponding to that of labradorite, 

 on the other hand, in the only meteorite in which its presence 

 has been established beyond doubt, is proved by Tschermak to 

 crystallise in the cubic system, instead of the anorthic system to 

 which terrestrial labradorite belongs. 



Attempts have been made to classify meteorites according to 

 their mineralogical constitution. As a provisional method, 

 such a classification has its uses ; but while we find that the same 

 meteorite may contain distinct portions which severally would 

 authorise its being placed in different classes, such a classification 

 must necessarily be very imperfect. 



The best general divisions are those of Gustav Rose ; and in 

 the following table are classed the various groups of Aerolites, 

 with a statement of the minerals that are met with in them : — 



Aerolites. 



Olivine. 



Bronzite. 



Augite. 

 ^ Nickel- Iron. 

 ' Troilite. 

 f Augite. 

 { Anorthite. 

 i Nickel- Iron. 

 f Bronzite or Enstatite. 

 I Augite (occasional). 

 / Nickel-Iron. 



Troilite Oldhamite (occasional). 

 ; Osbornite. 

 ) Chromite. 

 (J Olivine. 

 •• Chromite. 



Olivine. 



Enstatite. 



Nickel-Iron. 



Sulphur. 



Carbon. 



Troilite. 



Chromite. 



Hydrocarbons. 



The great division of meteorites into iron masses or siderites, 

 mixed masses or siderolites (the pallasites and mesosideritcs of 

 Rose), and aerolites or stony meteorites ; and tlie sub-division of 

 the latter into chondritic and non-chondritic varieties, seems to be 

 a sufficiently logical division. And among the non-chondritic aero- 

 lites, those designated in Gustav Rose's classification as Eukrites 

 form one well-marked group. They consist of anorthite mingled 



Chondritic 



EUKRITIC 



Chladnitic 



Chassignitic. 



Carbonaceous 



sometimes with augite in a crystallogranular admixture, with 

 nickel-iron, troilite, magnetic pyrites, a little olivine, .ind small 

 amounts of other minerals. The crystals of anorthite and the 

 augite in the eukritic meteorite of Juvinas have afforded satis- 

 factory goniometrical measurements, and been identified as re- 

 gards their crystalline forms — the former, as before mentioned 

 by V. von Lang, and the augite by Gustav Rose — with the 

 corresponding terrestrial minerals ; and it is the eukritic aerolites 

 which most closely resemble some of our volcanic rocks. 



The carbonaceous meteorites form another remarkable though 

 not a distinct group. In these we meet with minerals 

 which, if occurring in a terrestrial rock, would lead us to 

 ascribe to that rock an igneous origin ; they are the same 

 minerals that occur in other meteorites (olivine, enstatite, &c.), 

 but are associated with carbon and with a minute amount 

 of a white or a yellowish crystallisable matter, soluble in 

 ether and partly so in alcohol, and exhibiting the characters 

 and the composition of one or more hydrocarbonous bodies 

 with high melting points. Such an ingredient permeating a 

 rock on our globe would assuredly be accepted as a product 

 resulting indirectly from animal or vegetable existence. We 

 must be cautious, however, in the extending of this generalisation 

 to celestial hydrocarbons. It seems not at all improbable that 

 this singular ingredient of these otherwise stony and fire-formed 

 meteoric rocks may have been taken up by the mass subsequently 

 to its formation ; perhaps while passing through an atmosphere 

 of these hydrocarbonous substances in the form of a vapour. 

 The probability of this is enhanced by the smallness in the 

 amount (about 0*25 per cent, only) of the white soluble bodies 

 contained in the aerolite, and by the fact that the whole of it may 

 be dissolved out from a mass of considerable size by the direct 

 treatment of the solid aerolite by the boiling solvent, even 

 without previous pulverisation ; the substance, in short, mechani- 

 cally fills the pores of the aerolite, but does not appear to be 

 otherwise contained or entangled in the interior of the silicates 

 or of the compacter aggregations of these within the meteorite. 



The remaining divisions into which aerolites have been 

 grouped are less distinctly marked, and their boundaries less 

 fixed than those we have considered. In fact, a more compre- 

 hensive knowledge of all the varieties of meteorites and the 

 modes in which their constituent minerals may ht associated 

 is needed for our forming a complete classification of them, and 

 it is only necessary to make one observation in order to indicate 

 the importance of our being able thus to arrange together these 

 meteorites which are strictly comparable, and may be supposed 

 to have had a common or at least a similar origin and history. 



Such a classification is in fact a necessary preliminary to our 

 ever successfully dealing with the problem of the periodically 

 recurrent visitation to our earth of any particular class or group 

 of meteorites. And it is here that the great collections of 

 meteorites brought together in the National European Museums 

 already are, and promise in a far higher degree in the future to 

 be, so valuable. They offer the opportunities for the most 

 complete comparison and the widest induction that our limited 

 material admits of. 



It may thus be possible hereafter by their aid to trace such 

 a periodicity in the falls of meteorites of particular kinds as has 

 been established in the cases of several meteor showers ; or 

 again the accumulation of observations recording the directions 

 from which these bodies fall to the earth may enable us to 

 connect those of a particular class with some definite direction 

 that may indicate for these a common source in space. It may 

 be feared, however, that owing to the species of refraction which 

 their paths must undergo on entering the atmosphere, and the 

 great difficulty, if not impossibility, of obtaining very accurate 

 comparable parallactic observations of their paths, it will be 

 impossible to rely on any calculated elements of their ori)its before 

 approaching our planet. 



One of the difficulties confronting us in any endeavour to trace 

 them to their sources, lies in the near similarity of composition of 

 very large groups of them, such for instance as the entire group 

 of the chondritic aerolites, or again thjit of the siderites, a 

 similarity so close in each case as to render it difficult at first 

 to suppose that the masses belonging to either of these groups 

 originated under dissimilar conditions, or in widely sundered 

 regions of space. 



A difficulty of a similar kind further presents itself in the 

 relative importance of nickel as an ingredient in the iron element 

 of meteorites. One cannot indeed mstitute a comparison in this 

 respect with the iron of our globe, which cannot be said to exist 

 within the scope of our knowledge in the native state, while on 



