Marshall. — Geology of Tahiti. 361 



Art. XXXIX.— The Geology of Tahiti. 



By P. Marshall, M.A., D.Sc, F.G.S., Professor of Geology, Otago 



University. 



[Read before tht Otago Institute, 1st December, 1014.] 



Plate VII. 



All those islands of the Central Pacific which have been examined geologic- 

 ally are either formed of coral rock or they are of volcanic origin. No 

 sediments other than those that have the nature of coral detritus or of 

 volcanic tuffs are known, with the possible exception of the coal-seams 

 that are said to occur at the Island of Rapa. 



In a few of the islands the occurrence of plutonic rocks has been recorded. 

 Such records are, however, at the least, doubtful at Borabora, Maupiti, and 

 at the Marquesas. At Sunday Island, in the Kermadec Group, there are 

 certainly blocks of granite of considerable size and of some number em- 

 bedded in a volcanic breccia. 



In the Island of Tahiti, however, a series of plutonic rocks has been 

 definitely proved to exist. The first record of these appears to be contained 

 in Cuzent's work on Tahiti in 1860. Afterwards, in the year 1898, specimens 

 were found in old collections previously stored in " l'ancien musee colonial/' 

 in Paris. These, like Cuzent's specimens, had been collected in the valley 

 of the Papenoo, in which the Tuoru River flows. 



Professor Lacroix at once realized the interest attached to these rocks, 

 and in 1901 he induced M. Seurat to search for them. This distinguished 

 biologist was at that time sent on a zoological mission to Tahiti in connec- 

 tion with the pearl-shell industry. The success of M. Seurat's geological 

 work is described in the following words : ' M. Seurat a procede a, V explora- 

 tion herisee de difficultes de loute la vallee de Papenoo et particulierement de 

 sa partie haute. II est parvenu ainsi a trouver le gisement en place de la roche 

 en question. "* 



With the aid of"M. le Capitaine Courtet, who had at a previous time 

 made a survey of the valley, a good map was drawn, and on it the geological 

 information obtained by M. Seurat was inserted. This map was of the 

 greatest value, as it was in all respects more accurate and more detailed 

 than the official map used in Tahiti. So far as the lower course of the river 

 was concerned, it appeared to be absolutely correct. It was not until we 

 reached the upper part of the valley that any discrepancies were found 

 between the map and the actual courses of the streams. 



Professor Lacroix had hoped that the work of M. Seurat would have 

 enabled him to decide the vexed point as to whether the plutonic rocks 

 were the remnant of an ancient eroded land-mass, or whether they were 

 masses intrusive into the basaltic series of which the island is almost entirely 

 composed, in the same way as the gabbros are intrusive into the lavas of 

 the Hebrides. In this important respect the results were inconclusive, 

 for he says, " Malheureusement, je ne suis que poser ce probleme. M. Seurat 

 m'a dit n'avoir pas vu de blocs de couleur clair dans les tufs basaltiques, mais 

 les researches precises seraient necessaires pour elucider ce probleme."^ 



Matters resting in this rather unsatisfactory state, advantage was taken 

 of a visit to Tahiti in August, 1913, made with the aid of a grant from the 



* Lacroix, " Les roches alcalineti do Tahiti," Bull. Soc. Geol. de France, 4 e scrie, 

 t. x, 19:0, p. 92. 



t Lacroix, lor. fit., p. 97. 



