XIII SAVU-SAVU PENINSULA 191 



The basaltic andesites of the peninsula are often extensively 

 ■decomposed through the weathering process, a spheroidal structure 

 being then displayed. It rarely happens that the basaltic rocks of 

 this locality assume a propylitic character. Yet, if this change is 

 due to hydrothermal metamorphism, we ought to find altered rocks 

 ■of this kind in the vicinity of the boiling-springs. Such rocks did 

 not come under my notice at the surface ; but this only indicates 

 that if this alteration has taken place here, it has been effected at 

 •some depth ; and, indeed, it would seem probable that the alter- 

 ation known as " propylitic " is a change produced generally in 

 deep-seated rocks. 



A semi-ophitic basaltic andesite that is exposed in the small 

 stream-course at the back of the springs, and not 100 yards distant, 

 displays no propylitic change, and is only affected by hydration. 

 The basaltic andesites found on the hill-slopes further inland from 

 the springs exhibit no change of such a nature. However, rocks 

 of this description occur at and near the coast about a mile 

 to the westward. One of them, which is light green in colour, 

 inight be taken for a limestone, since it effervesces with an acid. 

 When examined in the slide it is shown to be the prevailing basaltic 

 andesite greatly altered. The porphyritic rhombic pyroxene is 

 replaced by viriditic material ; the plagioclase phenocrysts are 

 replaced by calcite, secondary silica, and other alteration products ; 

 and the structure of the groundmass is disguised by chalcedony, 

 calcite, viridite, &c. Another rock from this locality displays great 

 alteration. The structure of the groundmass is obscured by 

 secondary silica, and is traversed by fine cracks passing through 

 the felspar phenocrysts and filled with blood-red films of hematite. 



On the hill-slopes behind Harman's Point, at an elevation of 

 300 to 400 feet, blocks of a reddish, volcanic rock, greatly altered 

 by the deposition of silica, were displayed on the surface. The 

 ground was here strewn in places with beautiful pyramidal prisms 

 of clear quartz, ranging up to an inch in length. They contain 

 numerous inclusions, their faces being sometimes deeply etched or 

 eroded. These crystals appear to have been formed rather rapidly 

 in some highly siliceous thermal underground waters. 



I did not ascend the hills of the portion of the peninsula 

 lying east of Na Kama and Naithekoro. But whilst crossing the 

 saddle between these two places, I perceived that the prevailing 

 basaltic andesites extended up the slopes to the east. The 

 neighbourhood of Naindi Bay offers several features of interest. 

 The bay, which is circular in shape, is closed in on the east and 



