496 



NATURE 



{Aprils, 1877 



collecting together of the fragments previously separated 

 from one another in comets, and an examination of thin 

 transparent sections with high magnifying powers and 

 improved methods of illumination, proves still more 

 conclusively their brecciated structure. The facts are, 

 however, very complex, and some are not easily ex- 

 plained. Leaving this question for the present, I will 

 endeavour to point out what appears to be the very 

 earliest history of the material, as recorded by the internal 

 structure. 



It is now nearly twenty ytars since I first showed that 

 the manner of formation of minerals and rocks may be 

 learned from their microscopical structure. I showed 

 that when crystals are formed by deposition from water 

 or from a mass of melted rock, they often catch up por- 

 tions of this water or melted stone, which can now be 

 seen as cavities containing fluid or glass. We may thus 

 distinguish betYv^een crystalline minerals formed by purely 

 aqueous or by purely igneous processes ; for example, 

 between minerals in veins and minerals in volcanic lavas. 

 In studying meteorites it appeared to me desirable, in 

 the first place, to ascertain whether the crystalline m'- 

 nerals found in them were originally formed by deposition 

 from water or from a melted stony material analogous to 

 the slags of our furnace or the lava of volcanoes. One 

 of the most common of the minerals in meteorites is 

 olivine, and when met with in volcanic lavas this mineral 

 usually contains only a few and small glass cavities in 

 comparison with thofe seen in such minerals as augite. 

 The crystals in meteorites are, moreover, only small, and 

 thus the difficulty of the question is considerably in- 

 creased. However, by careful examination with high 

 magnif)ing power, I found well-marked glass-cavities, 

 with perfectly fixed bubbles, the inclosed glass being 

 sometimes of brown colour and having deposited crys- 

 tals. On the contrary I have never been able to detect 

 any trace of fluid- ravities, with moving bubbles, and 

 therefore it is very probable, if not absolutely certain, 

 that the crystalline minerals were chiefly formed by an 

 igneous process, like those in lava, and analogous volcanic 

 rocks. These researches require a magnifymg power of 

 400 or 600 linear. 



Passing from the structure of the individual crystals to 

 that of the aggregate, we find that in some cases we have 

 a structure in every respect analogous to that of erupted 

 lavas, though even then there are very curious differences 

 in detail. By methods like those adopted by Daubree, 

 there ought to be no more difficulty in artificially imitat- 

 ing the structure of such meteorites than in imitating 

 that of our ordinary volcanic rocks. It is, however, 

 doubtful whether meteorites of any considerable size uni- 

 formly possess this structure. The best examples I have 

 seen are only fragments inclosed in the general mass of 

 the Petersburg meteorite, which, like many others, has 

 exactly the same kind of structure as that of consolidated 

 volcanic tuff or ashes. This is well shown by the 

 Bialystock meteorite, which is a mass of broken crystals 

 and more complex fragments scattered promiscuously 

 through a finer-grained consolidated dust-like ash. 



Passing from this group of meteorites, which are more 

 or less analogous to some of our terrestrial volcanic 

 rock'^, we must now consider the more common varieties, 

 which are chiefly composed of olivine and other allied 

 minerals. The Mezo Madaras meteorite is an excellent 

 illustration, since the outlme of the fragments is well 

 seen, on account of the surrounding consolidated fine 

 material being of dark colour. In it we see more or less 

 irregular spherical and very irregular fragments scattered 

 promiscuously in a dark highly consolidated fine-grained 

 base. By far the larger part of these particles do not 

 either by their outline or internal structure furnish any 

 positive information re^pectmg the manner in which they 

 were formed, but careful examination of this and other 

 analcgf.us meteorites, has enabled me to find that the 



form and structure of many of the grains is totally unlike 

 that of any I have ever seen in terrestrial rocks, and 

 points to very special physical conditions. Thus some 

 are almost spherical drops of true glass in the midst of 

 which crystals have been formed, sometimes scattered 

 promiscuously, and sometimes deposited on the external 

 surface, radiating inwardly ; they are, in fact, partially 

 devitrified globules of glass, exactly similar to some arti- 

 ficial blow-pipe beads. 



As is well known, glassy particles are sometimes given 

 off from terrestrial volcanoes, but on entering the atmo- 

 sphere they are immediately solidified and remain as 

 mere fibres, like Pele's hair, or as more or less irregular 

 laminae, like pumice dust. The nearest approach to 

 the globules in meteorites is met with in some artificial 

 products. By directing a strong blast of hot air or steam 

 into melted glassy furnace slag, it is blown into spray, and 

 usually gives rise to pear-shaped globules, each having a 

 long hair-like tail, which is formed because the surround- 

 ing air is too cold to retain the slag in a state of perfect 

 fluidity. Very often the fibres are the chief product. I 

 have never observed any such fibres in meteorites. If 

 the slag be hot enough, some spheres are formed 

 without tails, analogous to those characteristic of 

 meteorites. The formation of such alone could hot 

 apparently occur unless the spray were blown into 

 an atmosphere heated up to near the point of fusion, 

 so that the glass might remain fluid until collected 

 in'o globules. The retention of a true vitreous condition 

 in such fused stony material would depend on bo:h the 

 chemical composition and the rate of cooling, and its 

 permanent retention would in any case be impossible if 

 the original glassy globule were afterwards kept for a 

 long time at a temperature somewhat under that of 

 fusion. The combination of all these conditions may very 

 well be looked upon as unusual, and we may thus explain 

 why grains containing true glass are comparatively very 

 rare ; but though rare they point out what was the origin 

 of many others. In by far the greater number of cases 

 the general basis has been completely divitrified, and the 

 larger crystals are surrounded by a fine-grained stony 

 mass. Other grains occur with a fan-shaped arrange- 

 ment of crystalline needles, which an uncautious, non- 

 microscopical observer might confound with simple con- 

 cretions. They have, however, a structure entirely 

 different from any concretions met with in terrestrial 

 rocks, as for example that of oolitic grains. In them we 

 often see a well-marked nucleus, on which radiating 

 crystals have been deposited equally on all sides, and the 

 external form is manifestly due to the growth of these 

 crystals. On the contrary the grains m meteorites now 

 under consideration have an external form independent of 

 the crystals, which do not radiate from the centre, but 

 from one or more places on the surface. They have, 

 indeed, a structure absolutely identical with that of some 

 artificial blowpipe beads which become crystalline on 

 cooling. With a little care these can be made to crystal- 

 lise from one point, and then the crystals shoot out from 

 that point in a fan-shaped bundle, until the whole bead is 

 altered. In this case we clearly see that the form of the 

 bead was due to fusion, and existed prior to the formation 

 of the crystals. The general structure of both these and 

 the previously described spherical grains also shows that 

 their rounded shape was not due to mechanical wearing. 

 Moreover, melted globules with well-defined outline could 

 not be formed in a mass of rock pressing against them on 

 all sides, and I therefore argue that some at least of the 

 constituent particles of meteorites were originally detached 

 glassy globules, like drops of fiery rain. 



Another remarkable character in the constituent par- 

 ticles of meteorites is that they are often mere fragments, 

 although the entire body before being broken may origin- 

 ally have been only one- fortieth or one-fiftieth of an inch 

 in diameter. It appears to me that thus to break such 



