XXIV PALAGONITE 



343 



crushing. The heat thus developed is sufficient to partly fuse the 

 glass ; but since it is not great, it only affects the most fusible 

 constituents, and the remelted material corresponds therefore to 

 the magma-residuum of the consolidating mass, which is referred 

 to in the previous paragraph. It has the same unstable characters 

 and the same tendency to assume the palagonitic condition. 



This theory centres around the relatively low fusibility of the 

 magma-residuum that gives rise to palagonite. This degree of 

 fusibility has yet to be ascertained, since according to the views 

 here advanced it may even be much lower than that of tachylyte. 

 It is, however, noteworthy that the melting-point of tachylyte is 

 far below that of the more crystalline basaltic rocks, since it can 

 be readily determined, as I have done in the instance of a dyke- 

 like mass penetrated by tachylyte- veins before referred to, that the 

 veins are composed of a much more fusible material than the 

 rock-mass. From a very crude experiment I would infer that the 

 melting-point of ordinary tachylyte is not much above that of lead 

 (335° C). The fusion-point of an ordinary hemicrystalline basalt, 

 according to the well-known experiments on the lavas of Vesuvius 

 and Etna, would probably be over 1,000 degrees C. 



Two interesting experiments, the one artificial, the other 

 natural, may be here cited in connection with this view. Bunsen ^ 

 more than half a century ago, as a result of some experiments in 

 which he produced palagonite, arrived at the conclusion that the 

 tuffs formed of this material are submarine deposits derived from 

 the breaking up of previously formed palagonite-masses. Having 

 obtained this substance by placing powdered basalt in an excess of 

 melted potash-hydrate (Kalihydrat) and then adding water to the 

 silicate of potash thus formed, he concluded that palagonite results 

 from the reaction between glowing augitic-lavas and rocks rich in 

 lime and other alkalies. Although Zirkel quotes in this connection 

 the example where this material has been produced in the Cape de 

 Verde Islands by basic lava flowing over limestone, he rejects 

 Bunsen's explanation as inapplicable to extensive palagonite 

 districts, such as occur in Iceland, though allowing that it would 

 account for the local production of this substance. 



I venture to think, however, that in these two experiments the 

 general principles involved in the production of palagonite are 

 partly illustrated. We may accept the results of an experiment 

 without acquiescing in its interpretation. As I take it, it is in the 

 partial wet fusion of the powdered basalt that the secret of this 



1 Quoted in Zirkel's Petrographie, iii., 689. 



