268 APPENDIX, NO. III. 



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 congelation 

 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. Believe 

 me, &c. C. DARWIN." 



" To Professor FORBES." 



Professor Forbes confirmed the previous remarks by others made 

 by himself on the specimens transmitted 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 ^Etna. 



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 laminae are rendered distinct 

 hy 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 crystallisation, or to concretionary action, we should here have had the finest 

 parallel zones of different composition and texture. In glaciers, the lines of porous 

 ice and of minute crevices seem to be due to an incipient stretching, 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 characterised 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 laminae 

 of different 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 apparent." 



