154 



THE VOYAGE OF H.M.S. CHALLENGER 



On the shore of the island, at the foot of the volcano, there is a girdle of coral 

 easily accessible at low tide. The polyps are fixed to the volcanic rock, and the top 

 of the bank rises a foot above sea-level. The island has thus at a comparatively 

 recent period been subject to oscillations such as may be expected in a volcanic region. 

 After these brief remarks on the geological phenomena of Goonong Api we shall 

 describe the lithological characters of the eruptive products collected on the top of the 

 volcano. 



We shall begin with the less decomposed lavas, and afterwards deal with those 

 which show in their altered appearance traces of the action of the acid vapours to which 

 they have been exposed. All these rocks belong to the type of augitic andesites. 



Some very slightly decomposed lavas are black, very lustrous, slightly scoriaceous, 

 and spotted with felspathic grains. Microscopically they are formed of a yellowish base 

 crowded with micro] iths of plagioclase and augite, and in this ground-mass are seen 

 rather large sections of plagioclase, augite, magnetite, and, as an accessory mineral, 

 olivine. 



The microporphyritic crystals of plagioclase, which are vitreous, like sanidine, are 

 sharply outlined, and are elongated following the edge, PjM, but in other cases they 

 are less tabular, assuming the prismatic form. The most common types of twinning 

 of these plagioclases are those of Baveno and of albite, but the hemitropic lamellae are 



not numerous in the sections. The felspar sections 

 often present the appearance of two halves joined 

 together, resembling at first glance a Carlsbad twin, 

 but closer examination almost always shows one or 

 two hemitropic lamellse — sometimes excessively thin 

 — enclosed in one or other of the principal individuals. 

 These striae prove that this felspar is plagioclase. 

 Fig. 27 shows a section of plagioclase from the 

 rock we are describing. The section is parallel to 

 the face M of one individual (I) and more or less 

 parallel to the face P (zone P:k) of the other (II). 

 It can be seen that (I) is traversed by cleavages 

 parallel to P, which are parallel to the plane of union 

 and to the plagioclastic stria? of (II). Cleavages 

 The angle of extinction approaches 40°. The indi- 

 vidual (II) exhibits less regular fractures, resembling those usually seen in sanidine. 

 It is noticeable that, as is almost always the case, only one of the cleavages following 

 the prism is to be seen. The angle of extinction for the principal individual, measured 

 from the intercalated polysynthetic lamellse, is about 30°. The extinctions on M and 

 P exceed those of bytownite, and are nearer to those of anorthite. The extinctions 



Fig. 27. — Decomposed lava of Goonong Api. 

 Section of plagioclase. 



parallel to T can also be seen. 



