Marshall. — Geology of Mangaia. 333 



Art. XLII. — Note on the Geology of Mangaia. 

 By P. Marshall, D.Sc, Professor of Geology, Otago University. 

 [Read before the Otago Institute, 16th November, 1909.] 



The island of Mangaia is situated in lat. 22° 30' S. and long. 157° 30' E. 

 It is thirty miles in circumference. Charles Darwin represents it merely 

 as an island with a fringing reef. It has since Darwin's time often been 

 stated that the island is mainly composed of raised coral rock. The follow- 

 ing account is taken from a description kindly forwarded to me by Major 

 Large, the Resident Agent on the island. 



The fringing reef s 100 to 300 yards broad, and on its outer side vises 

 steeply from deep water. The inner edge is bounded in most places at high- 

 water mark by overhanging masses of old coral. Sloping up from this is 

 a broken rocky tract, fairly level on top. The road which connects the 

 three coastal settlements is formed on this belt. From this there rises 

 another belt of coral rock with perpendicular clifis to a height of 60 it. to 

 80 ft. ; its width is a quarter of a mile, and it is bounded by steep cliffs 

 on its inner side. This is the makatea of the Natives. Inside the niakatea 

 there is a low-lying belt of rich soil, from which the central moimtain — the 

 crown of Mangaia — an irregular-shaped flat-topped hill, rises to a height 

 of about 660 ft. The surface of this is formed of hard volcanic clay. 



From this it appears that there have been two distinct movements of 

 elevation, when the two coral reefs were raised which now form the lings 

 of coral rock that form the outer part of the island. The low-lying ring of 

 country must have been the lagoon inside the first coral reef. The crown of 

 Mangaia, which consists of volcanic rock, is the old island. Major Large 

 kindly sent me specimens of coral rock and of ferruginous flints that are 

 found in it. and also specimens of volcanic rock from the crown of Mangaia. 



The ferruginous flints proved to be nothing more than silicified portions 

 of the coral rock, in which the structure of the coral is still quite distinct in 

 microscopic section. It appears that these flints were not us(:d for making 

 the Native weapons, because of their brittleness. 



Several pieces of rock were found to be Ijasaltic scoria in a more or less 

 decomposed state. A more solid piece of rock proved to be a basalt con- 

 taining relatively large phenocrysts of feldspar (labradorite). The rock 

 contained but little olivine. A Native adze was formed of a very fine- 

 grained rock, which p-oved to be a typical basalt. Examination of these 

 rocks shows that the island is composed of material quite different from that 

 of Rarotonga and Aitutaki, which I have previously described as composed 

 of alkaline varieties of volcanic rock. 



Since the above was written a visit to the island has shown me the 

 accuracy of Major Large's description. The flat-topped hill in the centre 

 appears to mark an ancient sea-level when the island was a submarine 

 shoal. Despite a long period of depression, during which the makatea was 

 formed, the summit of the island is now 660 ft. above its original level. 

 The rocks are extensively decomposed. I found nothing but basalt and 

 dolerites. 



