364 Transactions. 



In the bed of the Tahinu, about a mile above the point where the river 

 joins the Maroto, there is an outcrop of white syenite which extends for about 

 100 yards along the bed of the stream. This rock is seamed most irregu- 

 larly by thick and thin dykes of a dark rock of a diabasic nature. Two 

 hundred metres above the syenite outcrop the Tahinu again branches, and 

 the right-hand branch retains the name Tahinu. In the bed of this stream 

 no boulders of plutonic rock could be seen ; all the boulders had a basaltic 

 appearance. In the bed of the left-hand branch, the Terefaatautau, plutonic 

 boulders of various natures were quite frequent. This branch was followed 

 for a short distance, but no rock was seen in situ. The proportion of boulders 

 of plutonic rock slightly decreased, and the stream in this part of its course 

 was relatively open, and not enclosed in a gorge. 



The Maroto branch was not followed for some distance from its junction 

 with the Tahinu, but it was found that about half a kilometre above that 

 point it cuts through a plutonic rock (gabbro). A short distance farther 

 on a small tributary which joins it on the right contained no boulders of 

 basalt, but a great variety of large boulders of plutonic rock. Another 

 half a kilometre farther on the stream showed a large exposure of peridotite. 

 A little distance farther on there was close to its right bank an occurrence 

 of a highly porphyritic theralite containing large crystals of hornblende. 

 Above this point the stream contained as many boulders of basalt as of 

 plutonic rock. Nearly a kilometre farther on, and at a short distance 

 from the right bank, there was a large outflow of spring-water of a strongly 

 ferruginous character. The water was highly charged with a gas which had 

 no smell and would not # support combustion ; it was probably carbon dioxide. 



The rock near the spring was much decomposed ; it contained large 

 crystals, and appeared to be a theralite. This spring was said by my guide 

 to be Vai Apaaoa, which in Lacroix' map is placed on the left bank of the 

 Maroto. All the ground near this spring is saturated with spring-water, 

 and the soil is everywhere red with iron oxide. The water of a neighbouring 

 stream is as great in volume, and contains ferruginous matter to as great 

 an extent, as Vai Apaaoa, and it is evident that there is close at hand 

 another spring as large as the one that we saw. The water of these springs, 

 which is quite cold, tinges the whole of the water of the Maroto a ferruginous 

 tint. The water of the Tahinu is tinged in a similar manner near the out- 

 crop of syenite, and spring-water is everywhere escaping in some quantity 

 near the main stream. This evolution of spring-water may account for the 

 fact that the syenite contains a considerable quantity of pyrite. Above 

 its junction with the Terefaatautau the water of the Tahinu was quite clear, 

 and in the Terefaatautau there was very little discoloration due to spring- 

 water. It appears, then, that the evolution of spring-water occurs only in 

 the area of plutonic rock, and, so far as observed, the escape of the spring- 

 water was greatest near the margin of the plutonic rock. 



On the left side of the Maroto, opposite the spring Vai Apaaoa, no plutonic 

 jock was seen on the side of the hill which forms the watershed between the 

 Tuoru and the Tamanu. 



From the summit of this watershed at Tetiairi Pass a good view was 

 obtained of the whole of the central basin of the island. The general form 

 of this basin was now disclosed, and was seen to fully justify the expression 

 of Meinieke : "' Liegt er an der Westseite eines grossen runden Bergkranzes 

 tier das Thai <les obcrni Papenoo flusses umschliesst."* 



* Meinicke, " Inseln des stillen Oceans," Zweiter teil, p. 164. Leipzig, 1875. 



