Thomas. — On the Rocks of the Kermadec Islands. 315 



glass similar to the one just described. This glass cannot be 

 termed a tachylyte proper, for it does not gelatinise on digestion 

 with hydrochloric acid, though the acid decomposes it to a 

 certain extent, extracting a good deal of iron. It seems to be of 

 an intermediate character, and to be the glassy form of a rock of 

 less basic character than basalt, probably of augite-andesite. 



It has already been stated that, with one exception, the rocks 

 found on the Kermadecs were of distinctly volcanic origin. The 

 exception is formed by pebbles and boulders of a light-coloured 

 rock, found only on the beach at Sunday Island. It can be 

 recognized by the naked eye that the rock is a crystalline 

 granular aggregate of a dull white felspar, quartz, and horn- 

 blende. The microscope shows, in addition, grains of magnetite. 

 The felspars are chiefly orthoclase, but plagioclase is present in 

 smaller proportion. No trace of any ground-mass appears 

 between the crystalline elements, and the rock is therefore one 

 to which the name of hornblende-granite would naturally be 

 given. Another variety of the same rock is very poor in horn- 

 blende and richer in quartz, which shows a distinct granulitic, 

 or almost micro-granulitic, character. 



The occurrence of a granitoid rock on these islands in mid- 

 ocean is somewhat unexpected, and is one to which much 

 interest is attached, when considered in connection with specu- 

 lation as to the former extension of land in the Pacific Ocean, 

 especially as the Kermadecs are situated along the submarine 

 ridge which stretches out from New Zealand to the north-north- 

 east. The fact that the rock is found only in detached masses 

 on the beach, suggests the question whether they can be of 

 foreign origin ; whether, for instance, they have been brought 

 as ballast by some ship. The number of the boulders, and their 

 size, (attaining a diameter of 2 feet or more,) forbids this 

 supposition. Until more information is gained by the further 

 examination of the islands, the most probable explanation of 

 their presence on the Kermadecs is to suppose them to be por- 

 tions of deep-seated rocks, possibly rocks which represent lavas 

 which have solidified at great depth from the surface, and that 

 they have been torn off from below and brought up to the 

 surface during an eruption. If this be the case, blocks of the 

 rock may yet be found, either embedded in the lava streams or 

 lying in the volcanic tuffs of the islands. 



