Marshall. — Geology of Tubuai Islands and Pitcairn. 279 



Poe Pounder, from Rapa Island. — In hand-specimens a coarse rock in 

 which feldspar, augite, and olivine can be easily distinguished. In section 

 the feldspar is found to have occasionally a large angle of extinction — as 

 much as 40°- — and thus is a basic type of labradorite. The great majority 

 of the crystals, however, have very narrow lamellae, and extinguish at angles 

 below 20°. The species apparently varies from andesine to basic labradorite, 

 which is confirmed by the low index of refraction, xiugite is present in large 

 ophitic plates with the pleochroism that is so common in the titaniferou* 

 varieties. Olivine is present in large irregular grains slightly serpentinized, 

 and it is often associated with minute flakes of brown mica. Iron-ore is 

 abundant. It is mainly ilmenite, as shown by the shape of the crystals 

 and by the frequent border of leucoxene. There is much apatite, especi- 

 ally as inclusions in the plates of augite. 



I have lately received from Mr. G. R. B. Christian, the Chief Magistrate 

 of Pitcairn, a box of rock-specimens from that remote spot. He states that 

 the specimens sent represent the various kinds of rock that are to be found on 

 the island so far as can be judged by external appearance. My only 

 previous reference to rocks of this island is a statement that a specimen 

 given to me by Mr. G. W. Card was a glassy hypersthene andesite.* The 

 rock-specimens that I have now received cause me to correct the previous 

 classification. I now recognize the rock as a glassy basalt. In the specimen 

 previously examined only one small crystal could be seen. It was thought 

 to be hypersthene, but subsequent specimens now show that it was olivine 

 stained slightly with iron oxide. The more numerous specimens that I 

 now have are all fine basalts, many of which are glassy. The feldspar is 

 an acid labradorite. Olivine, usually in idiomorphic crystals, is usually 

 more plentiful than the colourless augite, which is always granular. The 

 fineness and frequently glassy nature of the rocks suggest that they have 

 a submarine origin. So far as these rock-specimens are concerned, they 

 indicate that on several of the islands of the Tubuai Group a similar rock 

 type occurs. Tubuai, Rapa, Rurutu, and Pitcairn at least have highly 

 feldspathic basalts. In general all of the rock-specimens that I have 

 examined are so similar that they all might have occurred on the same 

 island. The specimens from Tubuai and Rapa are types that have been 

 selected by the natives for the manufacture of weapons, and hence they 

 may possibly be unusual types on the islands, selected because of their 

 special fineness or toughness. The specimens from Rurutu and Pitcairn, 

 however, appear to be typical of the rock occurrences on those islands. 

 Generally it may be said that the rocks of the Tubuai Group appear to be 

 less basic than those of Tahiti and the Cook Islands, and the specimens so 

 far examined show none of the alkaline characters found in many of the 

 rocks from those groups. 



* Handbuch der regionalen Geologie, Bd. vii, Abt. 2, p. 14, 1912. Michel Levy 

 (Examination petrographique de quelques roehes volcaniques des iles Tuamotu et de l'ile 

 Pitcairn, C. R. Acad. Sci. de Paris, cxli, p. 895-97, 1905) writes as follows: '"En resume 

 il existe a Pitcairn et a Mangareva deux series de roehes basaltiques : une plus acide de 

 basaltes andesitiques passant a des andesites a olivine et a des tachylites (Pitcairn): 

 une autre plus basique composee de basaltes labradoriques quelquefois tres augitique 

 d'autrefois tres riche en olivine du premier temps : cette derniere parait etre la plus 

 frequente. II faut voter en outre l'existence a File Pitcairn de ponces tracbitiques 

 presque entierement vitreuses et par suite peu interessantes au pointe de vue niine- 

 ralogique." The more basic type mentioned by Michel Levy was not represented in my 

 specimens. 



