XXIV PALAGONITE 



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 

 Assures which thus present the appearance of being occupied by 

 veins of pitchstone. 1 a <-"p>cu oy 



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' 

 1 to 3 mches thick, of a tachylytic glass which begins to fuse 

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

 sometimes contain palagonite. The basalt, penetrated by the 

 veins, has a smoky gro.mdmass displaying imperfect felspar-lathes 

 n a feebly refractive glassy base and containing a few small 

 magma-lakelets" that cannot be distinguished from palagonfre' 

 Near the mouth of the Narengali valley (see page .49) I found 

 what appears to be a palagonite-tuff overlain by agglomerates 

 formed of tachylytic pitchstone and of semi-vitreous amygdaloid" 

 basalts. The tuffconsists of fragments of a brown basic glass the 

 larger 1 to 2 rmlhmetres in si 2 e, carrying porphyritic plagioc ase 



paLonit r e The n f°*> ** ^^ *** « 1Ied ^ 

 palagonite The glass fragments possess the eroded mar™, 



indicated ,„ the accompanying figure. It may be remarked tha 



his type of tuff differs from that of the prevailing palagonite-tuff 



Indln LT ^ VaCU °' ar ' ■'" tHe 3bSenCe ° f —i-e organic remains, 

 and in its homogeneous composition. It is described on pa^e ,u 

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

 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 

 palagomtisation has taken place in the veins of basaltic gas 

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

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

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

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

 group of facts I venture to propose this theory. 



1 Zirkel's PetrograpMe, iii, 694. 



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



In this connection see the description „f, he So Ioa-levu pitchstone on p. 3 , 2 . 



