138 A NATURALIST IN THE PACIFIC CHAP, 



parallel to the flow. Chalcedonic flints, some of them showing 

 the agate-structure, together with fragments of silicified corals, are 

 found occasionally on the surface in this district. 



THE LAMBASA PLAINS 



These remarkable inland plains, about ten miles long and three 

 to five miles broad, are well described in the Admiralty chart as 

 a low undulating country covered with grass, screw-pines, and 

 Casuarina trees. They are backed by the mountains forming the 

 central axis of the island, whilst broken groups of hills, not usually 

 more than 500 or 600 feet in height and attaining in Ulu-i-Mbau 

 an elevation of 1,160 feet, intervene between them and the sea- 

 border. They are traversed by the Wailevu, Lambasa, and Ngawa 

 rivers which after breaking through the seaward hill-ranges pass 

 through broad mangrove-belts to reach the coast. The tide 

 ascends these river-courses for several miles ; and in the case of 

 the Lambasa river boats can follow its winding course for ten 

 miles penetrating into the heart of the plains. Much of this level 

 inland region is less than 100 feet above the sea ; whilst the contour 

 line of 300 feet by which the region is defined in the map attached 

 to this book fairly well indicates the higher levels. 



The features which we have described in the instances of the 

 Sarawanga and Ndreketi plains are in the main reproduced in the 

 Lambasa region ; but in the last-named each of the three rivers 

 has a system of hot springs along its course, namely (as described 

 in Chapter III.), at Na Kama on the Wailevu River, at Vuni-moli 

 on the Lambasa River, and at Mbati-ni-kama on the Ngawa River. 

 Basaltic andesites, often exposed at the surface in a decomposing 

 condition, form the foundation of the plains. They are overlain 

 by submarine clays containing pteropod shells and tests of fora- 

 minifera ; and over these in their turn coarse palagonitic tuffs and 

 agglomerate-tuffs are found in places. Formations still more recent 

 are represented by elevated reef-rock on the seaward side of the 

 hills that bound the plains. Nodules of chalcedony, silicified 

 corals, and other siliceous rocks, together with fragments of impure 

 limonite, lie on the surface over much of this region. 



The basaltic rocks of this region rarely show olivine, and belong 

 as a rule to the basaltic andesites, being referred to genera 13 and 

 21 of the augite-andesites, the specific gravity being about 2*8. 

 The felspar-lathes, *I2 to -14 mm. in length, are in flow-arrange- 

 ment, and the augite is at times semi-ophitic ; whilst there is a little 



