XVI MOUNT THUKU 231 



inland they are also to be observed. T he streams have worn deep 

 gorges into their mass. Towards Thuku they are acid and pumice- 

 ous, and are evidently the products of eruption. Towards Thawaro, 

 they are more basic and darker, and are in part at least to be 

 attributed to marine degradation. 



(d) Mount Thuku. — My ascent of this hill, which is 1,288 feet 

 in height, was made from the north coast. I found it to be com- 

 posed from the foot to the summit of white pumiceous tuffs without 

 any evident arrangement. It has a narrow top and shows no sign 

 of a crateral cavity. The hills east and west, as viewed from its 

 summit, are ridge-shaped and display nothing in their configur- 

 ation at all suggestive of craters. The pumice-tuffs of Mount 

 Thuku are non-calcareous, and exhibit greyish pumiceous lapilli in 

 an abundant white matrix formed of fine pumice-debris. Under 

 the microscope it shows the characteristic vacuolar and fibrillar 

 structure ; but the material has not the fresh appearance of ordin- 

 ary pumice and the minute cavities are often filled with alteration 

 products. The two rocky points on the north coast opposite the 

 hill are formed in one case of a somewhat altered oligoclase- 

 trachyte and in the other of a quartz-porphyry. Both no doubt 

 represent intrusive masses, the almost horizontal columns, 12 to 

 1 5 inches in diameter, of the former indicating a nearly vertical 

 dyke. 



(3) The Undu Promontory East of Mount Thuku 



East of Mount Thuku the hilly backbone of the promontory 

 is of much less elevation. About three miles to the eastward the 

 highest hill is 630 feet, and thence to within a mile or two of 

 Undu Point the hills retain a height of 400 to 500 feet. 



(a) The north coast between Mount Thuku and the coast village 

 of Nuku-ndamu. — On this stretch of coast, about five miles in 

 length, the shore-cliffs are composed of white and pale-yellow, 

 coarse and fine stratified pumice-tuffs, the beds being either hori- 

 zontal or with a gentle dip northward. They are as a rule non- 

 calcareous, and contain some quartz grains and small bits, i to 3 

 miUimetres in size, of bottle green compact obsidian, much as one 

 finds in Lipari pumice-tuffs. In general character, both naked-eye 

 and microscopic, they correspond to the Mount Thuku pumice- 

 tuffs above described. Large blocks of basic rocks are occasion- 

 ally to be observed on this coast, sometimes probably indicating 

 dykes, but in one place near Mount Thuku forming an agglomerate. 



