Marshall. — Geology of Tahiti. 373 



of the world, inserts a query as to the possibility of carbonate rocks being 

 cut by the eruptive rocks of Tahiti.* 



All round this island the ocean-floor quickly falls to a depth of about 

 2,500 fathoms, and its surface is covered with volcanic mud, or with this 

 material mixed with globerigina ooze or with red clay.f There appears to 

 be no reason to suppose that before volcanic activity commenced on the 

 site of Tahiti the ocean-floor at that particular spot was in any way different 

 from its nature in the closely adjacent area. Over this area the ocean- 

 floor is at such a depth that the calcium-carbonate remains of organisms 

 have already been partly dissolved and the percentage of silica has become 

 considerable. 



The question also arises as to whether any instances are known where 

 basic magmas have invaded limestone and have acquired in consequence 

 a local alkaline character on the borders of the magma. So far as New 

 Zealand is concerned, we do not at present know of any district in this 

 country where basic magmas have invaded limestone rocks. There is, 

 however, a striking instance of a large acid batholite in contact with a 

 limestone deeply buried at the time of the intrusion of the magma. This 

 example has been described by Webb, Bell, and Clarke. J Here the great 

 intrusive mass of granite of the Pikikiruna Boss is in contact with a crystal- 

 line limestone of Ordovician age. Basic rocks are the result of the contact, 

 and patches of basic rock due apparently to absorption and solution of 

 limestone are found at some distance within the granite. There appears 

 to be no evidence, notwithstanding the obvious evolution of quantities of 

 carbonic-acid gas, of any great disturbance of the magma, or of any 

 influence of the absorbed limestone, on the whole mass of the granite, 

 but only on localized portions of it. No development of alkaline types is 

 noticed. Similarly, in the district of alkaline volcanic rocks at Dunedin, 

 New Zealand, where a highly arenaceous limestone of Cainozoic age under- 

 lies the volcanic rocks, no evidence has so far been found that any solution 

 of this material has taken place. 



In the actual field occurrence of the plutonic rocks in the Papenoo 

 Valley, Tahiti, the occurrence of acidic types and ultra-basic types within 

 a short distance of one another appears to preclude the idea of absorption 

 of calcareous matter. There is every reason to believe that the outcrop is 

 at a level of at least 10,000 ft. to 15,000 ft. above that at which absorption 

 has to be supposed. Upward movement through this distance would cause 

 the complete mixing of the absorbed matter. Yet here we find a complete 

 separation of the magma into rocks of which one at least cannot be regarded 

 as a desilicated type. 



The rock-specimens found on the beach at Tautira and in the gravels 

 of the Tautira Biver included gabbros and phonolites. The Tautira Kiver 

 is the largest in the Taiarabu Peninsula, and, judging by the map, it appears 

 to have a large circular basin in the centre of the peninsula. There can 

 be no doubt that this peninsula was an independent volcano, and the 

 Tautira Biver appears to have the same relation to it as the Papenoo has 

 to the main island. In other words, the Tautira appears to have drained 

 the original crater, and now that denudation is far advanced its basin includes 



*Daly, "The Origin of Alkaline Rocks," Bull. Geol. Soc. Amer., vol. 21, 1910, 

 pp. 87-118. 



t Agassiz, Mem. Mus. Conip. Zool., vol. 28, plates, pi. 202. 

 % N.Z. Geol. Surv. Bull. No. 3 (new series), 1907, p. 73. 



