84 A NATURALIST IN THE PACIFIC chap, 



comes upon a tract strewn with large blocks, amongst which occur 

 fragments of huge columns 3 to 4 feet in diameter ; but it is on the 

 steep southern slopes of the plateau in the vicinity of Ndavutu and 

 Masusu that the most extensive exposures of columnar basalt are 

 to be found. Here there have been large clearings made for the 

 tea-plantations, and portions of columns 2 to 3 feet in thickness 

 are scattered all over the slopes and surface of Masusu. 



A very interesting exposure occurs on the southern edge of 

 the Masusu flat facing Ulu-i-ndali. Here there is displayed in the 

 face of a waterfall a mass of basalt about 40 feet deep, formed of 

 regular cross-jointed columns, 3 to 4 feet in diameter and often 

 pentagonal in shape, which are almost perpendicular, being inclined 

 about five degrees from the vertical. But in the upper portion of 

 the fall the columns are smaller (2 to 3 feet across) and become 

 arched and nearly horizontal. This was the only section of the 

 inner mass of the basaltic flows that I found, and here the columns 

 are almost vertical. In this locality several other exposures of 

 the columnar basalt occur ; but they are all at the surface and the 

 columns are nearly horizontal or very much inclined from the 

 vertical, being often pentagonal in form, 2 to 3 feet across, and 

 sometimes curved with joints 10 to 20 feet in length. 



Neither vesicular nor scoriaceous rocks came under my notice 

 in this region, and the presence of pteropod-ooze deposits and of 

 foraminiferous clays and tuffs on the slopes of the basaltic table- 

 land indicates that the flows were submarine. The common 

 character of a sub-aerial basaltic flow, where there are large 

 vertical columns below and smaller radiating columns above, did 

 not present itself ; and it is probable that the singular arrangement 

 of the columns in the upper portion of these flows may be con- 

 nected with the conditions of depth under which the flows took 

 place. 



It is apparent from the description given by Dana of the 

 columnar basalt of Tahiti^ that it was formed under different 

 conditions from those under which the basaltic flows of Wainunu 

 and Seatura were formed. The columns composing a cliff 500 

 feet high in the Matavai valley were 10 to 20 inches across. A 

 bluff, 200 to 300 feet high, in another part of the valley, was made 

 up of columns 5 to 8 inches in width. The tallest cliff displayed 

 in places converging and curved columns, which is attributed to 

 the unequal cooling of the interior of the mass ; but it is evident 

 from a diagram given by the author that the columns were not 

 ^ Geology of the United States Exploring Expedition. 



