XXIV PALAGONITE 



347 



{d) In submarine eruptions are to be found the conditions 

 favouring the production of palagonite on a large scale. In the 

 case of such basaltic flows it is probable that their upper portions 

 are formed entirely of palagonite arising from the alteration of a 

 vitreous magma-residuum extruded on the surface in the manner 

 above described. Such a crust, as a result of shrinkage and other 

 processes, would probably present itself to the geologist as a some- 

 what friable material, passing gradually into the overlying sub- 

 marine deposits. 



Note on the type of basalt found associated with palagonite. 



The type is characterised, it would appear, rather by its 

 structural features than by its mineral composition. It is the 

 basalt of ophitic or semi-ophitic habit that would seem to be 

 usually associated with palagonite ; and since this habit is as a 

 rule to be found where the groundmass displays large felspar-lathes 

 in plexus arrangement, coarse augites, and at least a fair amount 

 of smoky glass, it follows that a hemi-crystalline, ophitic or 

 semi-ophitic, doleritic basalt is the type to be associated with 

 palagonite. 



This is the type of rock that forms the lower part of the 

 basaltic flow near Kiombo, the upper part of which is largely 

 palagonitic. To this structural type also belong most of the 

 basalts in my collection where palagonite exists in the form of 

 " magma-lakelets " in the groundmass. These " lakelets " are 

 almost diagnostic of this type of basalt. Here also belongs the 

 famous globular basalt of Acicastello on the coast of Sicily .^ In 

 such rocks the felspar-lathes form a mesh-work and vary usually 

 in average length between -i and '3 mm. The augites of the 

 groundmass, typically semi-ophitic, range up to 'i mm. in size. 

 They are always large, that is, over -03 mm., and this coarseness 

 is another important indication. 



1 I have visited this locality on several occasions with the special object of 

 studying the relation of the basalt to the associated palagonite-tuffs and clays. 

 A general discussion of this question would be out of place here ; but I may 

 remark that the conclusion arrived at by me is that these deposits are not sedi- 

 mentary but are entirely the result of the disintegration of palagonite in situ. 

 This is quite opposed to the view of their sedimentary origin held by Dr. 

 Johnston-Lavis, Prof. Platania, and other Itahan geologists. . . . The basalt 

 is scoriaceous, semi-vitreous, and semi-ophitic, and closely approaches the type 

 of basalt above defined. 



