34° A NATURALIST IN THE PACIFIC chap. 



smoky, partly devitrified, glass. They are usually more or less opaque 

 and reddish-brown or yellowish in colour, whilst they have often 

 a marked zoned structure, the concentric bands conforming to the 

 irregular contours of the lakelet. In the least affected stage the 

 zones show fibrous devitrification across their breadth, but as the 

 palagonitic change progresses the material becomes opaque. In 

 the secondary changes, such as those associated with the early 

 alteration of the propylites, these " magma lakelets " are the first 

 affected. They then present alternating layers of calcite and 

 viridite and are often bordered by magnetite. 



If these "lakelets" were to be described as collections of 

 residual glass, the description would be insufficient, since they 

 may occur in the midst of a smoky, partially devitrified, glass. 

 During the last stage in the consolidation of the basaltic mass, 

 the magma-residuum that still retains its fluidity collects here and 

 there in the crevices of the groundmass, and forms little pools of 

 usually microscopic dimensions into which the felspar-lathes often 

 protrude from the sides. These little pools or lakelets represent 

 that portion of the yet fluid magma that during the last stage of 

 consolidation is imprisoned in the stiffening mass — like the whey 

 in a cheese — whilst the greater part of it has been squeezed into 

 the cracks of the cooling mass, as occurs in a dyke-like intrusion 

 below described, or has been extruded on its surface, as in the case 

 of the basaltic flow above referred to. 



As a suggestive instance of the formation of palagonite " in 

 situ," I will now refer to a basic tuff-agglomerate on the plateau 

 of Na Savu (see p. 81) which is penetrated by veins, a few inches 

 thick, apparently composed of a finely brecciated pitchstone-tuff. 

 In the section the material forming the veins is seen to be 

 composed of fragments of basic glass (carrying porphyritic 

 plagioclase and augite) which have been crushed in position, the 

 interspaces being filled up with the finer debris of the glass and 

 of the minerals together with palagonitic material. The glass 

 fragments, which have lost their sharp edges and angles, are often 

 palagonitised at the borders, and we thus get a patch of isotropic 

 brown glass with a yellowish margin formed of a feebly refractive 

 turbid substance. Where this border is not so evident, it is noticed 

 that the edge of the glass is peculiarly eroded. The indication 

 appears to be that the fissures in this agglomerate were filled with 

 a basic magma that after its solidification into a glass was 

 subjected to a crushing process, and that during this process a 

 partial remelting of the glass took place which resulted in the 



