MR. DARWIN ON THE STRUCTURE OF LAVAS. 267 



APPENDIX, No. III. 



EXTEACTS from LETTERS to PROFESSOR FORBES, on the Analogy 

 of the Structure of some Volcanic Kocks with that of Glaciers. 

 By C. Darwin, Esq., F.R.S. With Observations on the same 

 subject, made by PROFESSOR FORBES.* 



" I take the liberty of addressing yon, knowing how much you 

 are interested on the subject of your discovery of the veined structure 

 of glacier ice. I have a specimen (from Mr. Stokes's collection) of 

 Mexican obsidian, which, judging from your description, must resemble, 

 to a considerable degree, the zoned ice. It is zoned with quite straight 

 parallel lines, like an agate ; and these zones, as far as I can see 

 under the microscope, appear entirely due to the greater or lesser 

 number of excessively minute, flattened air cavities. I cannot avoid 

 suspecting that in this case, and in many others, in which lava of the 

 trachytic series (generally of very imperfect fluidity) are laminated, 

 that the structure is due to the stretching of the mass or stream 

 during its movement, as in the ice-streams of glaciers. * * 



" If the subject of the lamination of volcanic rocks should interest 

 you, I would venture to ask you to refer to p. 65-72 of my small 

 volume of * Geological Observations on Volcanic Islands/ f I there 



* [Proceedings of the Eoyal Society of Edinburgh, 3d February 1845. There 

 ought to have been a reference to this paper at page 92 of this volume.] 



f The laminated, volcanic rocks of Ascension, consist, as described by Mr. Dar- 

 win, of excessively thin, quite parallel layers of minute crystals of quartz (deter- 

 mined by Professor Miller) and diopside ; of atoms of an oxide of iron, and of an 

 amorphous, black angitic mineral ; and, lastly, of a more or less pure feldspathic 

 stone, with perfect crystals of feldspar placed lengthways. The following is a 

 portion of the passage referred to: " Several causes appear capable of producing 

 zones of different tension in masses semi-liquified by heat. In a fragment of de-vi- 

 trified glass I have observed layers of sphserulites, which appeared, from the manner 

 in which they were abruptly bent, to have been produced by the simple contraction 

 of the mass in the vessel in which it cooled. In certain dykes on Mount .ZEtna, 

 described by M. Elie de Beaumont, as bordered by alternating bands of scoriaceous 

 and compact rock, one is led to suppose that the stretching movement of the sur- 

 rounding strata, which originally produced the fissures, continued, whilst the injected 

 rock remained fluid. Guided, however, by Professor Forbes's clear description of 

 the zoned structure of glacier ice, far the most probable explanation of the laminated 

 structure of these feldspathic rocks appears to be, that they have been stretched, 

 whilst slowly flowing onwards in a pasty condition, in precisely the same manner, 

 as Professor Forbes believes, that the ice of moving glaciers is stretched and fissured. 

 In both cases, the zones may be compared to those in the finest agates ; in both, 



