346 A NATURALIST IN THE PACIFIC chap. 



it is also evident that the mode of junction between the flow and 

 the overlying deposits is of an unexpected nature. Before drawing 

 any inferences, it is necessary to point out that when we begin on 

 a priori grounds to frame our notions as to the course of events 

 on the surface of a submarine basaltic flow, we are entering a little 

 known region of inquiry. I would, however, suggest in the light 

 of the theory before advanced, the following explanation of the 

 appearances presented by this series. 



During the consolidation of the flow much of the magma- 

 residuum that still retained its fluidity was extruded on the sur- 

 face, where after solidification it became palagonitised. Accord- 

 ing to my view this would be the typical behaviour of submarine 

 basaltic flows ; but, owing to the unstable and perishable nature of 

 the palagonitic crust of the flow, it would be rarely preserved in 

 upheaved volcanic regions. There would probably be, as in the 

 case of the Nandua series, no sharp line to be drawn between the 

 palagonite-crust and the deposits subsequently covering it, deposits 

 indeed that would derive no inconsiderable proportion of their 

 materials from the disintegration of the crust itself. During and 

 after the emergence of such a district of submarine eruptions the 

 unstable palagonitic crust would be further subjected to the 

 hydration resulting from weathering and similar agencies ; and as 

 a result of its final degradation there would often alone remain a 

 bed of reddish argillaceous material. 



In concluding these remarks on palagonite the following 

 summary of the principal points here dwelt upon may be 

 added : 



(a) The basic glass, that undergoes the palagonitic change, is 

 the vitreous form of the magma-residuum that in a particular 

 type of basalt and under certain conditions remains fluid after the 

 mass of the rock has solidified. During the last stage of the con- 

 solidation it is in part imprisoned in the " magma-lakelets " of the 

 groundmass ; whilst the rest of it is squeezed into cracks and 

 fissures, or extruded on the surface of the flow. 



(b) This glass differs from ordinary basic glass in its molecular 

 condition, its mineral composition, its low degree of fusibility, and 

 in its unstable character. 



(c) The formation of palagonite in connection with the crush- 

 ing of a basic glass is to be explained by the hypothesis that the 

 heat developed during the crushing is sufficient to partially re-fuse 

 the glass, the material thus produced corresponding to the magma- 

 residuum of low degree of fusibility, which is above referred to. 



