CHAPTER XXIII 



CALCAREOUS FORMATIONS, VOLCANIC MUDS, PALAGONITE-TUFFS 



THE classification that is adopted in my work on the geology of 

 the Solomon group with respect to the calcareous formations and 

 volcanic muds of those islands is only in part applicable to the 

 calcareous rocks and volcanic deposits of Vanua Levu. Deposits 

 strictly comparable with those of the Solomon Islands here exist, 

 and have in some places an extensive distribution ; but many 

 others cannot be referred to that classification. In addition to the 

 calcareous oozes and volcanic muds, such as are now forming off 

 these reef-bound coasts, the result partly of marine erosion and 

 partly of sub-aerial denudation, there are many kinds of submarine 

 deposits in Vanua Levu that have been largely formed from the 

 materials ejected by volcanic vents. Basic glasses, for instance, 

 often finely vesicular and usually converted into palagonite, enter 

 largely into the composition of submarine deposits that frequently 

 form the surface from the sea-borders to the summits of the 

 mountain-ranges ; and it is by the degradation of a land-surface 

 formed of such materials that the volcanic muds comparable to 

 those of the Solomon Islands are mainly produced. It is therefore 

 apparent that we have to distinguish here between the deposits of 

 sedimentary and eruptive origin, a distinction, however, which is 

 not always easy to make, since they are in both cases submarine, 

 and doubtless were often in process of forming together. The 

 deposits most prevalent in the island are the submarine tuffs partly 

 sedimentary and partly eruptive in their origin and the overlying 

 volcanic agglomerates. The first are usually palagonitic and 

 calcareous and often contain organic remains, being usually 

 associated with volcanic muds and clays mainly the product of 

 marine erosion. 



