Art. XLII.—Note on the Geology of Mangaia.
[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 following account is taken from a description kindly forwarded to me by Major Large, the Resident Agent on the island.
The fringing reef is 100 to 300 yards broad, and on its outer side rises 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 cliffs to a height of 60 ft. 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 makatea there is a low-lying belt of rich soil, from which the central mountain—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 rings 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 used for making the Native weapons, because of their brittleness.
Several pieces of rock were found to be basaltic scoria in a more or less decomposed state. A more solid piece of rock proved to be a basalt containing relatively large phenocrysts of feldspar (labradorite). The rock contained but little olivine. A Native adze was formed of a very finegrained rock, which proved 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.