18 



throw out the idea, that the structure in question may perhaps be 

 explained by your views on the zoned structure of glacier ice, the 

 layers of less tension being, in the case of the Ascension obsidian- 

 rocks, rendered apparent, chiefly by the crystalline and concretionary 

 action superinduced in them, instead of, as in zoned ice, by the con- 

 gelation of water. * * * 



" How singular it at first appears, that your discoveries in the 

 structure of glacier ice should explain the structure, as I fully believe 

 they will, of many volcanic masses. I, for one, have for years been 

 quite confounded whenever I thought of the lamination of rocks which 

 have flowed in a liquified state. Will your views throw any light 

 on the primary laminated rocks ? The laminse certainly seem very 

 generally parallel to the lines of disturbance and movement. Be- 

 lieve me, &c. C. Dakwin." 



To Professor Forbes. 



Professor Forbes confirmed the previous remarks by others, made 

 by himself on the specimens ti'ansmitted to him by Mr Darwin, and 

 on specimens from Lipari and Iceland in the collection of the Royal 

 Society, as well as by direct observations made by himself on the 

 lava streams of .^tna. 



distinct by the subsequent congelation of infiltrated water ; in the stony 

 feldspathic 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 crystalliza- 

 tion, or to concretionarj' 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 feldspathic 

 lavas with horizontal laminae, 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 feldspathic lavas, divided into lamiuse of differ- 

 ent composition and texture. Moreover, in the augitic 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." 



