Marshall. — Geology of Moorea and Rurutu Islands. 283 



to closely resemble the coarse dolerite that is so common in Rarotonga, 

 Tahiti, Huaheine, and Raiatea. Unfortunately, I could not obtain a speci- 

 men of this rock for microscopic examination. On the west side of the 

 island the material at the sea-level is a fine-grained igneous rock similar to 

 the fragments contained in the calcareous agglomerate on the eastern side. 

 This rock contains enough magnetite to deflect the compass-needle in the 

 most marked manner. Under the microscope it is found to contain rela- 

 tively large crystals of magnetite embedded in a very fine-grained ground- 

 mass consisting largely of minute microlites of labradorite feldspar, some 

 very small crystals of olivine, minute granules of augite and of magnetite. 

 Though extremely fine-grained, there is no glass whatever in the rock, 

 which has evidently been formed from the very rapid cooling of basaltic 

 lava. 



From the statements made to me by Mr. Hildreth, combined with the 

 results of my examination of the rock-specimens, the following appears to 

 be the geological history of the island : Submarine volcanic action first 

 formed the agglomerate on a sea-floor that was covered with calcareous ooze. 

 The eruptions continued, and formed a volcano of moderate size. The old 

 sea-floor was afterwards raised until it attained an elevation of 300 ft. above 

 sea-level. There is apparently a well-marked terrace at an elevation of 

 about 300 ft. on the east side, which probably marks a sea-level that 

 remained unchanged for a considerable period. 



Art. XXXIX. — The Geology of the Cape Runaway District. 



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



University. 



{Read before the Otago Institute, 2nd December, 1913.] 



The strip^of country running due east and west from Cape Runaway to 

 Matakaoa Point has not yet received much attention from the geological 

 standpoint. Throughout this twenty miles of coast-line the country rises 

 steeply, and soon reaches to an elevation of 1,300 ft. The country slopes 

 down nearly as steeply on the southern side of the ridge into the valleys 

 of the Wharekahika and Ngarue Streams, running east and west respectively. 

 Thus this narrow bar of country is marked off from all that lying to the south, 

 and constitutes a physiographical unit. In a north-north-east direction the 

 ocean-floor dips down steeply, and within fifty miles there is a depth of 

 1,000 fathoms, and within two hundred miles the ocean is 4,380 fathoms 

 deep. This profound abyssal depth marks the farthest known extension of 

 that great earth-feature the Kermadec-Tonga trench ; but, since soundings 

 are relatively few in this part of the Pacific Ocean, it is quite possible that 

 future work will show that this trench actually extends still farther to the 

 south. This is the nearest known approach of abyssal depths to the coast- 

 line of New Zealand. The average slope throughout this distance of two 

 hundred miles is about 137 ft. per mile — that is, 1 in 40. 



