580 H. B. GUPPY ON THE RECENT 



calcareous islands the greatest height at which the soft deposits occurred 

 was 1150 feet, which was the greatest elevation at which I found any of the 

 recent calcareous formations in this group. They probably occur in the western 

 end of Choiseul at an elevation of 1500 feet and over. 



EXPLANATION OF PLATES CXL1V., CXLY. 



Heights in feet ; depths in fathoms. Arrows indicate the dip ; horizontal beds 

 shown by a cross, thus + . 



Plate CXLIV. 



Fig. 1. Plan of Ugi Island, \ inch to a mile. 



Fig. 2. Ideal section of Ugi Island, intended to illustrate the relative thinness of the coral 



limestone. 

 Fig. 3. Plan of Treasury Island, £ inch to a mile. 



Fig. 4. Ideal section of Treasury Island, adapted to show its principal structural features. 

 Fig. 5. Plan of Santa Anna Island, £ inch to a mile. 



Fig. 6. Section of Santa Anna Island, drawn on a true scale of 2 inches to a mile. 

 Fig. 7. Diagram showing the relative position of the raised coral rock and its foundation to the mass 



of volcanic rock in situ on the reef flat ; north coast of Santa Anna Island. 

 Fig. 8. Ideal section of Alu Island, showing the relation to each other of the coral limestone, the soft 



Pteropod and Foraminiferous deposit, and the volcanic rock. 



Plate CXLV. 



Fig. 1. Section of a compact grey Globigerina limestone from Treasury' Island (see page 576). The 

 rock contains 66-26 per cent, of carbonate of lime, consisting chiefly of species of pelagic 

 Foraminifera, such as Globigerina bulloides, G. hirsuta, G. (Orbulina) universu, Pullenia 

 obliquiloculata, and Pultinulina menardii ; Amphislegina, and one or two other bottom 

 living Foraminifera. The organisms are cemented by carbonate of lime, sometimes 

 crystallised in rather large grains (d). The mineral particles consist of (1) plagioclase- 

 felsjtar, presenting the extinctions of labradorite; sometimes these crystals are twinned 

 following the albitic law (c); sometimes they are twinned following the law of pericline. 

 The crystal (e) shows a plagioclastic section, exhibiting at the same time the twinning, 

 following the law of albite and pericline ; (2) fragments of hornblende ; one of these (a) is 

 cut perpendicular to the crystallographic axis, and presents the hexagonal contours, and the 

 cleavage lines crossing at an angle of 124°; (3) fragments of augite; the section (b) of this 

 mineral presents two individuals grouped with their axes parallel; the section is octagonal, 

 and crossed by the cleavages at right angles ; (4) fragments of magnetite and biotite. 



Fig. 2. Section of a tufa of an augite-andesite, from the central region of Treasury Island (set.' page 

 551). The rock contains 7 - 74 per cent, of carbonate of lime, which consists of a few 

 coccoliths, several species of Globigerina, and fragments of Echinoderms. The ground- 

 mass is composed of particles of volcanic glass, magnetic particles, and small decomposed 

 mineral fragments. The larger minerals are lamellae of plagioclase (d), twinned following 

 the albite law, with the extinction of labradorite; also twinned, following the law of 

 Baveno (a); sections of augite (//) more or less parallel to the axis, with a twinning following 

 the orthopinakoid ; and crystals of augite (c) with a black glass particle attached, pro vim; 

 that the crystal was not formed in situ; also some hornblende. The presence of the GloU- 

 gerincB and Echinoderm particles show that the tufa was formed under water in which the 

 surface organisms lived. 



