80 Lamination of Volcanic Rocks *art i. 



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. 1 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 laminae are rendered 

 distinct by the subsequent congelation of infiltrated 

 water, in the stony feldspathic lavas, by subsequent 

 crystalline and concretionary action. Tbe fragment of 

 glassy obsidian in Mr. Stokes' collection, which is zoned 

 with minute air-cells, must strikingly resemble, "judging 

 from Professor Forbes'" descriptions, a fragment of the 

 zoned ice ; and if the rate of cooling and 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 forms and towards 

 the lower end of most glaciers, the zones become hori- 

 zontal. May we venture to suppose that in the feld- 

 spathic lavas with horizontal laminae, we see an analo- 

 gous case ? All geologists, who have examined trachytic 

 regions, have come to the conclusion, that the lavas of 



1 I presume that this is nearly the same explanation which Mr. 

 Scrope had in his mind, when he speaks (' Geolog. Transact.' vol. ii. 

 second series, p. 228) of the ribboned structure of his trachytic rocks, 

 having arisen, from ' a linear extension of the mass, while in a state 

 of imperfect liquidity, coupled with a concretionary process.' 



