XXIV PALAGONITE 



341 



molecular change characteristic of palagonite. Since the unaltered 

 glass-fragments fuse in the ordinary flame, it would seem that the 

 heat developed during the crushing might be sufficient to partially 

 remelt the glass without affecting the rock penetrated by the 

 veins ... It is of importance to note that in the palagonite-tuffs 

 of the Canary Islands the change is often most complete along 

 fissures, which thus present the appearance of being occupied by 

 veins of pitchstone.^ 



In this connection allusion may be made to a dyke-like mass of 

 a rubbly semi-vitreous basaltic rock exposed at Vatu-lele Bay, 

 described on page 184. It is penetrated in all directions by veins, 

 I to 3 inches thick, of a tachylytic glass which begins to fuse 

 in the ordinary flame. The glass is traversed by cracks which 

 sometimes contain palagonite. The basalt, penetrated by the 

 veins, has a smoky groundmass displaying imperfect felspar-lathes 

 in a feebly refractive glassy base and containing a few small 

 ■" magma-lakelets " that cannot be distinguished from palagonite.^ 



Near the mouth of the Narengali valley (see page 149) I found 

 what appears to be a palagonite-tuff overlain by agglomerates 

 formed of tachylytic pitchstone and of semi-vitreous amygdaloidal 

 basalts. The tuff consists of fragments of a brown basic glass, the 

 larger i to 2 millimetres in size, carrying porphyritic plagioclase, 

 and fractured in position, the interspaces being filled with 

 palagonite. The glass fragments possess the eroded margins 

 indicated in the accompanying figure. It may be remarked that 

 this type of tuff differs from that of the prevailing palagonite-tuffs 

 in being rarely vacuolar, in the absence of marine organic remains, 

 and in its homogeneous composition. It is described on page 334 

 under the head of " crush-tuffs." Whether it is derived from the 

 destruction of a mass of basic glass that had previously undergone 

 crushing and partial palagonisation I cannot say ; but its 

 characters point in the direction of this conclusion.^ 



In the foregoing pages it has been attempted to show that 

 palagonitisation has taken place in the veins of basaltic glass 

 traversing in one case a basic tuff agglomerate and in another case 

 an intrusive basaltic mass, and that it has also occurred in the 

 upper vitreous portion of a basaltic flow and in the materials now 

 composing a so-called "crush-tuff." In order to explain this 

 group of facts I venture to propose this theory. 



^ Zirkel's Petrographie, iii., 694. 



2 This basalt is not fusible in the ordinary blow-pipe flame. 



^ In this connection see the description of the Soloa-levu pitchstone on p.312. 



