OF THE DEPENDENT ISLES OF TAIWAN. 17 



III. Petrography of the EfFusives. 



The groundwork of the Pescadores is essentially built up of 

 Basalts, making extensive flows to the water's edge, and the whole 

 is encircled by the fringing reefs of cornls, which, in parts raised 

 above the water, connect many of the detached rocks with the shore, 

 thus contributing greatly to the enlargement of the areas of the 

 island. Each and every island visited by Tada and Saito, presents 

 the same physiognomy, and consists of the same black rock. The 

 specimens, brought back from most of the islands, and of which 

 descriptions will be given in the sequel, have a certain common 

 feature which stamps them as genetically identical, and their 

 field relations in different areas seem to point to a common 

 centre of volcanic activity. They exhibit, however, a considerable 

 variation of character. Thus from the same island, T have 

 specimens at one place perfectly massive and compact, at another 

 vesicular and porous, and sometimes Doleritic. Colours var}^ 

 from black to bluish-grey in fresh ones, and through weathering 

 the Doleritic and vesicular varieties become whitish or grey, 

 while the compact rocks acquire a reddish brown tinge. 



AVe are indebted to Mr. Y. Saito, for characterising the 

 different flows, and for tracing their vertical as well as horizontal 

 distributions in the Northern group. According to him, there 

 are three distinct Basaltic flows of nearly the same distribution, 

 separated by long time-intervals which are represented in inter- 

 bedded sedimentary rocks. Judging from the nearly perfect 

 horizontality which the beds and flows keep in all the islands, 

 it seems probable that there existed a lava field or volcanic mesa 

 of considerable extent. But, on account of its remote age, pro- 



