chap. in. of the Tr achy tic SeiHes. 77 



produced during the movement of the mass. 1 From 

 such facts, most authors have attributed the lamination 

 of these volcanic rocks to their movement whilst lique- 

 fied. Although it is easy to perceive, why each separate 

 air-cell, or each fibre in pumice-stone, 2 should be drawn 

 out in the direction of the moving mass ; it is by no 

 means at first obvious why such air-cells and fibres 

 should be arranged by the movement, in the same 

 planes, in laminae absolutely straight and parallel to 

 each other, and often of extreme tenuity ; and still less 

 obvious is it, why such layers should come to be of 

 slightly different composition and of different textures. 

 In endeavouring to make out the cause of the 

 lamination, of the igneous feldspathic rocks, let us 

 return to the facts so minutely described at Ascension. 

 We there see, that some of the thinnest layers are 

 chiefly formed by numerous, exceedingly minute, though 

 perfect, crystals of different minerals ; that other layers 

 are formed by the union of different kinds of concre- 

 tionary globules, and that the layers thus formed, often 

 cannot be distinguished from the ordinary feldspathic 

 and pitchstone layers, composing a large portion of the 

 entire mass. The fibrous radiating structure of the 

 sphagrulites seems, judging from many analogous cases, 

 to connect the concretionary and crystalline forces : the 

 separate crystals, also, of feldspar all lie in the same 



1 ' Geological Transactions,' vol. ii. (second series) p. 200, &c. 

 These embedded fragments, in some instances, consist of the lami- 

 nated trachyte broken off and ' enveloped in those parts, which still 

 remained liquid.' Beudant, also, frequently refers, in his great work 

 on ' Hungary ' (torn. iii. p. 386), to trachytic rocks, irregularly spotted 

 with fragments of the same varieties, which in other parts form the 

 parallel ribbons. In these cases, we must suppose, that after part 

 of the molten mass had assumed a laminated structure, a fresh 

 irruption of lava broke up the mass, and involved fragments, aud 

 that subsequently the whole became relaminated. 



2 Dolimieu's « Voyage,' p. 64. 



