chap. in. Lamination of Trachytic Rocks, 75 



zones appearing to contain a larger proportion of silica. 

 In another part of the island, there are layers of pearl- 

 stone and pitchstone, which in many respects resemble 

 those of Ascension. The zones in the columnar trachyte 

 are generally contorted ; they extend uninterruptedly 

 for a great length in a vertical direction, and apparently 

 parallel to the walls of the dike-like mass. Von Buch 1 

 has described at Teneriffe, a stream of lava containing 

 innumerable, thin, plate-like crystals of feldspar, which 

 are arranged like white threads, one behind the other, 

 and which mostly follow the same direction. Dolimieu 2 

 also states, that the gray lavas of the modern cone of 

 Vulcano, which have a vitreous texture, are streaked 

 with parallel white lines : he further describes a solid 

 pumice-stone which possesses a fissile structure, like 

 that of certain micaceous schists. Phonolite, which I 

 may observe is often, if not always, an injected rock, 

 also, often has a fissile structure ; this is generally due 

 to the parallel position of the embedded crystals of 

 feldspar, but sometimes, as at Fernando Noronha, seems 

 to be nearly independent of their presence. 3 From 

 these facts we see, that various rocks of feldspathic 

 series have either a laminated or fissile structure, and 

 that it occurs both in masses, which have been injected 

 into overlying strata, and in others which have flowed 

 as streams of lava. 



The laminae of the beds, alternating with the obsi- 

 dian at Ascension, dip at a high angle under the moun- 



1 ' Description des lies Canaries,' p. 184. 



2 ' Voyage aux Isles de Lipari,' pp. 35 and 85. 



3 In this case, and in that of the fissile pumice-stone, the struc- 

 ture is very different from that in the foregoing cases, where the 

 laminae consist of alternate layers of different composition or texture. 

 In some sedimentary formations, however, which apparently are 

 homogeneous and fissile, as in glossy clay-slate, there is reason to 

 believe, according to D'Aubuisson, that the laminas are really due to 

 excessively thin, alternating, layers of mica. 



