A CATALOGUE OF OBSERVATIONS OF LUMINOUS METEORS. 139 



Fig. 116. A fluid-cavity in the quartz of the main mass of granite at 

 Aberdeen, x 2000. 



Fig. 117. A stone-cavity in the quartz of the granite at St. Austell, Corn- 

 wall, x 1000. 



(7.) On the Microscopical Structure of Meteorites. 



By H. C. Sorby, F.R.S., &c. 

 (From the Proceedings of the Royal Society, June 16th, 1864.) 



" For some time past I have endeavoured to apply to the study of meteorites the 

 principles I have made use of in the investigation of terrestrial rocks, as described 

 in my various papers, and especially in that on the microscopical structure of crystals 

 (Quart. Journ. Geol. Soc. 1858, vol. xiv. p. 453. See the preceding Article). I 

 therein showed that the presence in crystals of ' fluid-, glass-, stone-, or gas- 

 cavities ' enables us to determine in a very- satisfactory manner under what con- 

 ditions the crystals were formed. There are also other methods of inquiry still 

 requiring much investigation, and a number of experiments must be made which 

 will occupy much time ; yet, not wishing to postpone the publication of certain 

 facte, I purpose now to give a short account of them, to be extended and completed 

 on a subsequent occasion*. 



"In the first place it is important to remark that the olivine of meteorites 

 contains most excellent 'glass-cavities,' similar to those in the olivine of 

 lavas, thus proving that the material was at one time in a state of igneous 

 fusion. The olivine also contains ' gas-cavities,' like those so common in volcanic 

 minerals, thus indicating the presence of some gas or vapour (Aussun, Parnallee). 

 To see these cavities distinctly, a carefully prepared thin section, and a magnify- 

 ing power of several hundreds are required. The vitreous substance found in the 

 cavities is also met with outside and amongst the crystals, in such a manner as to 

 show that it is the uncrystalline residue of the material in which they were 

 form id ( Mezo-Madaras, Parnallee). It is of a claret or brownish colour, and pos- 

 sesses the characteristic structure and optical properties of artificial glasses. Some 

 isolated portions of meteorites have also a structure very similar to that of stony 

 lavas, where the shape and mutual relations of the crystals to each other prove 

 that they were formed in situ, on solidification. Possibly some entire meteorites 

 should be considered to possess this peculiarity (Stannern, New Concord), but the 

 evidence is by no means conclusive, and what crystallization has taken place in situ 

 may have been a secondary result ; whilst in others the constituent particles have 

 all the characters of broken fragments (L'Aigle). This sometimes gives rise to a 

 structure remarkably like that of consolidated volcanic ashes, so much, indeed, that 

 I have specimens which, at first sight, might readily be mistaken for sections of 

 meteorites. It would therefore appear that, after the material of the meteorites 

 was melted, a considerable portion was broken up into small fragments, subsequently 

 collected together, and more or less consolidated by mechanical and chemical ac- 

 tions, amongst which must be classed a segregation of iron, either in the metallic 

 state or in combination with other substances. Apparently this breaking up oc- 

 curred in some cases when the melted matter had become crystalline, but in others 

 the forms of the particles lead me to conclude that it was broken up into detached 

 globules whilst still melted (Mezo-Madaras, Parnallee). This seems to have been 

 the origin of some of the round grains met with in meteorites ; for they occasion- 

 ally still contain a considerable amount of glass, and the crystals which have been 

 formed in it are arranged in groups, radiating from one or more points on the ex- 

 ternal surface, in such a manner as to indicate that they were developed after the 

 fraa-nients had acquired their present spheroidal shape (Aussun, &c). In this they 

 differ most characteristically from the general type of concretionary globules found 

 in terrestrial rocks, in which they radiate from the centre ; the only case that I 

 know at all analogous being that of certain oolitic grains in the Kelloways rock 

 at Scarborough, which have undergone a secondary crystallization. These facts 

 are all quite independent of the fused black crust. 



* " The names given thus (Stannern) indicate what meteorites I more particularly refei 

 to in proof of the various facts previously slated. 



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