Structure of Volcanic Rocks and Glaciers. 371 
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. * * 7 
“If the subject of the lamination of volcanic rocks should interest 
you, L 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 layers of minute crystals 
of quartz (determined by Professor Miller) and diopside ; of atoms of an 
oxide of iron, and of an amorphous, black augitic mineral ; and, lastly, 
of amore or less pure felspathic stone, with perfect crystals of felspar 
placed lengthways. The following is a portion of the passage referred to : 
—‘‘ Several causes appear capable of producing zones of different tension 
in masses semiliquified by heat. In afragment of devitrified glass I have 
observed layers of spherulites, 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 Attna, 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 
uid. 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 felspathic rocks appears to be, that they 
haye 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, 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 lamine are rendered 
distinct by the subsequent congelation of infiltrated water ; in the stony 
felspathic lavas by subsequent crystalline and concretionary action. 
The fragment of glassy obsidian in Mr Stokes’s collection, which is zoned 
with minute air-cells, must strikingly resemble, judging from Professor: 
Forbes’s description, a fragment of the zoned ice; and if the rates of 
cooling and the nature of the mass had been favourable to its erystalliza- 
tion, or to concretionary action, we should here have had the finest pa- 
rallel zones of different composition and texture. In glaciers, the lines of 
porous ice and of minute crevices seem to be due to an incipient stretch- 
ing, caused by the central parts of the frozen stream moving faster than 
the sides and bottom, which are retarded by friction. Hence, in glaciers 
of certain form, and towards the lower end of most glaciers, the zones 
become horizontal. May we venture to suppose that, in the felspathic 
lavas with horizontal laminee, we see an analogous case? All geologists 
who have examined trachytic regions have come to the conclusion, that 
the lavas of this series have possessed an exceedingly imperfect fluidity ; 
and as it is evident that only matter thus characterized would be subject 
to become fissured, and to be formed into zones of different tensions, 
in the manner here supposed, we probably see the reason why augitic 
lavas, which appear, generally, to have possessed a higher degree of 
fluidity, are not, like the felspathic lavas, divided into lamine of differ- 
_ ent composition and texture. Moreover, in the augitie series, there 
never appears to be any tendency to that kind of concretionary action, 
which, we have seen, plays an important part in the lamination of 
rocks of the trachytic series, or, at least, in rendering that structure ap- 
parent.” 
