342 



A NATURALIST IN THE PACIFIC CHAP. 



In certain types of basaltic lava, 1 when cooling and consolidation 

 take place under peculiar conditions, such as we would expect to 

 find in submarine eruptions, there is a residuum of the magma 

 with relatively low fusibility that remains fluid after general 

 solidification of the mass is well advanced. As the rock continues 

 to consolidate, portions of this magma residuum become imprisoned 

 in the mass, like whey in a cheese, giving rise to the " magma 

 lakelets " above described ; whilst other portions, during the 

 contraction and fissuring accompanying the cooling process, are 

 squeezed out into the cracks thus formed, or are intruded on the 

 surface of the consolidating mass, as in the case of a submarine lava- 

 flow. This solidified magma-residuum differs from the ordinary 



Showing fragments of glass with eroded borders and of plagioclase with more even edges 

 in a matrix of palagonite traversed by cracks. The length of the largest fragment 

 is half a millimetre. The glass has been evidently fractured in position and this is 

 true of one of the felspar fragments. It is also apparent that whatever its cause the 

 erosion of the margins of the glass has been produced since the fracture. 



basic glass not only in its lower degree of fusibility but in its 

 mineral composition and in its molecular condition. It probably 

 in the first place does not differ much in appearance from the 

 typical glass, but it is an unstable substance and is capable under 

 certain hydro-chemical conditions of developing the characters of 

 palagonite. 



In those cases where the occurrence of palagonite is associated 

 with evidence of crushing, the process appears to be in a sense 

 reversed, since partial palagonitisation of an ordinary basic glass 

 takes place as a result of the elevation of temperature due to the 



1 See the note at the end of this chapter. 



