74 A NATURALIST IN THE PACIFIC chap. 



being about 2-g. They approach in characters the grey porphyritic 

 olivine-basalts of the northern part of Seatura (pages 65, 66) ; but 

 differ amongst other features in the greater abundance of the 

 olivine and in exhibiting flow-structure. They are usually almost 

 holocrystalline, and are assigned for the most part to genus 14 of 

 the olivine-basalts. They are extensively exposed in the stream- 

 courses on the west side between 500 and 900 feet ; and huge 

 masses of the same rocks, but containing less olivine and more glass, 

 and displaying much calcite, viridite, and other alteration products, 

 are found near the base of the eastern slopes. The semi-vitreous 

 condition of these rocks is represented in the large masses of a 

 dark very scoriaceous porphyritic lava, possessing quite a cindery 

 appearance, that occur on the narrow ridge-shaped summit. The 

 groundmass shows a few scattered felspar microliths ; but it is in 

 the main composed of a dark opaque glass. Small cube-like 

 crystals of chabazite line some of the cavities. 



Other basic rocks are not infrequent and apparently represent 

 dykes. Thus on the eastern side at 800 feet is exposed a dark- 

 grey semi-ophitic doleritic rock (sp. gr. 277) assigned to genus 12 

 of the augite-andesites (page 275). The felspar-lathes average '3 

 mm. in length, and there is a little interstitial glass containing 

 viriditic and calcitic alteration products, the same materials filling 

 small rounded vesicular cavities. On the same slope between 1,000 

 and 1,200 feet, there are displayed fresh-looking compact non- 

 porphyritic basaltic andesites (sp. gr. 2'84), where the felspar-lathes 

 average -2 mm. and the interstitial glass is scanty. They are 

 referred to genus 16, species C, of the augite-andesites. On this 

 side also between 600 and 800 feet occur blocks of a highly altered 

 slightly vesicular augite-andesite showing a little microporphyritic 

 plagioclase. It is assigned to genus 13, species B, of the augite- 

 andesites. In one place where it is in position it is scoriaceous, the 

 steam-holes being round, empty and one to five mm. in size. In the 

 less glassy rock it displays numerous small irregular cavities either 

 filled with fibrous viridite or calcite or showing concentric zones of 

 the two minerals. The felspar-lathes are ■I5--2 mm. in length. In 

 blocks near the foot of the eastern slope occur a blackish olivine- 

 basalt (sp. gr. 2-88) of the prevailing Seatura type, possessing a 

 little interstitial glass and felspar-lathes with an average length of 

 ■2 mm. It belongs to genus 25 of the olivine-rocks. . . . On the 

 western slopes at a height of 500 feet occurs a dark compact rock 

 (sp. gr. 2'89) with abundant olivine which is referred to genus i of 

 the olivine basalts. There is a little residual glass, the felspar-lathes 



