^v VISONGO 



221 



from the vicinity of Kalikoso to the sea. At one place about 3 

 miles up the river a dyke of trachyte, much altered and showing 

 <:olumns 12 to 18 inches across, was exposed at the bank. 



(3) Sea-border between the Langa-langa River and 



Thawaro Bay 



The next part of the coast which I visited was that opposite 

 Tutu Island. Here a range of high hills, 1,100 or 1,200 feet in 

 height, sends a lofty spur to the sea, in the precipitous faces of 

 which are exposed breccia-tuffs and agglomerates derived from 

 acid rocks. A specimen of the tuffs shows, besides fragments of 

 altered rhyolitic or trachytic rocks, portions of decomposing 

 pumice, the vacuoles and tubular cavities of which are filled with 

 alteration products. The blocks in the agglomerates are altered 

 oligoclase-trachytes, both compact and vesicular. These deposits 

 are non-calcareous and rarely display bedding ; but in one place 

 there was a rude arrangement of the materials, the dip being to the 

 north-west. I crossed this coast range where it is only 600 feet 

 above the sea, and descended into the Kalikoso plains in the 

 vicinity of Numbu. 



In the coast district between Tutu Island and the village of 

 Naua, 3^ miles to the east, the same altered coarse pumiceous and 

 trachytic tuffs, occasionally bedded and dipping W.N.W., form the 

 low hills near the sea. In one place, where the elevation was less 

 than 200 feet, I noticed on the surface small fragments of branch- 

 ing corals. They had perhaps weathered out of the tuffs, and 

 though in part silicified still effervesced freely in an acid. 



Between Naua and the town of Visongo occur low hills formed 

 of acid tuffs. At an elevation of 200 feet the tuffs display a remark- 

 able character. When examined with the lens they are seen to be 

 crowded with the minute tests of foraminifera of the " Globigerina " 

 type. The deposit forms a fairly hard light-grey clay-rock which 

 according to the usual acid-test has no lime. Under the micro- 

 scope it is seen to be composed nearly in equal proportion of the 

 tests of foraminifera and of very fine detritus apparently derived 

 from acid rocks, the materials being generally not over 'Oi mm. in 

 diameter, though some of the felspar fragments measure '05 mm. 

 across. A secondary process of silicification, as exhibited in the 

 occurrence of minutely granular quartz, has affected the matrix of 

 the clay as well as the tests of the foraminifera. The deposit is of 



