48 KOTO : NOTES ON THE GEOLOGY 



Fringing reefs are said to skirt the shore, some portion attain- 

 ing doable the man's height above the water's edge, indicative 

 of a recent negative shift of the relative levels. It seems to me 

 probable, that they are not the reefs of Neocene time, which 

 usually attain a considerable height of more than 200 m., as in the 

 Apes Hill of Takao, but those of a comparatively recent date, 

 possibly representing a Diluvial formation. The plateau-like 

 elevation, which faces the sea in cliffs, seems in parts at least 

 in the north-east point to consist of volcanic agglomerate. A 

 greater part of the interior seems to be built of volcanic rocks 

 with a gabbro-like plntonic mass as the foundation of tlie island 

 exposed at the west coast, but their mutual relations and area 

 of distribution are quite unknown to me. 



In the following, I will give a succinct account of rocks, 

 kindly placed at my disposal by Messrs. Ishii and Torii. 



A. FELSPAR-BASALTS. 

 {PL III, Figs. 3 and 4.) 



My slides of Basalts and Andésites are prepared from chips 

 of water worn gravels, used as weights attached to a fishing net 

 of the aborigines. 



The Basalt is ratlier porous, greyish- brown mass with a few 

 phenocrysts of a brown olivine (1-2 mm.) and black common 

 augite. Under the microscope, the olivine occurs in two gene- 

 rations {Fig. 4). Its forms are acute six-sided, sometimes 

 nearly square, truncated at corners, but mostly corroded and 

 disfigured, with a few traces of basal cleavage. The crys- 

 tals are slightly decomposed in their margin, being either yellow 



