2. Extracts from Letters to the General Secretary, on the 

 Analogy of the Structure of some ^'olc•anic Rocks with 

 that of Ghiciers. By C. Darwin, Esq., F.R.S. Specimens 

 were exhibited. With Observations on the same sub- 

 ject, made by Professor Forbes. 



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

 are interested on the subject of your discovery of the veined struc- 

 ture 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 can- 

 not 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.'* I there 



* The laminated, volcanic rocks of Ascension, consist, as described 

 by Mr Darwin, of excessively thin, quite parallel laj-crs of minute crystals 

 of quartz (determined bj- 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 lo : 

 — *' Several causes appear capable of producing zones of ditTerent ten sion 

 in masses semiliquified by heat. In a fragment of devitrified glass I have 

 observed layers of sphterulites, 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 ^tna, described by M. Elie de Beaumont, as bordered 

 by alternating bands of scoriaceous and compact rock, one is led to sup- 

 pose that the stretching movement of the surrounding strata, which ori- 

 ginally produced the fissures, continued, whilst the injected rock remained 

 fluid. Guided, however, bj' Professor Forbes's cle;ir 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 past^- condition, 

 in precisely the sjime 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, they extend in 

 the direction in which the mass has flowed, and those exposed on the 

 surface are generally vertical. In the ice, the porous lamina) are rendered 



VOL. II. B 



