OF THE DEPENDENT ISLES OF TAIWAN. 29 



in this rock. The polysomatic mineral has a smaller index of 

 refraction, when compared with that of the accompanying pla- 

 gioclase, as may be easily experimented upon by Becke's method. 

 These colourless patches, as a rule, behave optically isotropic ; 

 at times, however, faintly double-refractive, and separate into 

 several optical fields. They readily dissolve in HCl, with the 

 formation of the cubes of rock-salt. The same patches frequently 

 resolve themselves into a radial-fibrous, somewhat brownish and 

 highly double-refractive body (well seen at the margin in the 

 lower, left quadrant in PL II, Fig. 5) with the positive sign along 

 the axis of the needles. 



The polygonal base-like mineral, moulded upon plagioelase 

 and augite, seems to be identical with what Biiking^^ calls the 

 ^ Basis zweiter Art,' and is allied to i\\Q pitchstone-glass of Hunter 

 and Rosenbusch.'-^ Recently, this base was studied with great zeal 

 by the American petrologists, Lindgren,"^ T. F. Williams,^' Kemp,^^ 

 Fairbanks,*^^ Cross,'- Coleman^* and Pirsson"'; the last author 

 especially paid close attention to this subject, in making care- 

 ful analyses and also recalculating tlie analytical result, ob- 

 tained by Hunter. From his study, Pirrson is forced to the 

 conclusion that the so-called colourless base has exactly the 



1) 'Basaltische Gesteine, etc.,' Jahrb. K. K. preus?. geol. Landesanstalt. 1S80, S. 153j und 

 1881, S. 606. 



2) ' Ueber Monchiquite, ein camptonitisches Ganggesteiu aus der Gefolgenschaft der 

 Eleolithsyenite,' Tschermark's Min. Math. XI, 1890, S. 415. 



3) Proc. Cal. Acad. Sei., Vol. HI, 1390. 



4) Cited in Pirsson's paper. 



5) 'Trap Dikes,' Bull. 107, U. S. G. S. 1893. 



6) 'On Aualcite Diabase from San Luis Obispo County, California,' Bull. Geol. Depart. 

 Univ. Cal., Vol. I. p. 27?. 



7) ' An Analcite-Basalt from Colorado,' Journ. Geol. Vol. V. p. 684. 



8) 'A new Analcite Kock from Lake Superior' Journ. Geol. Vol. VII, 1899, p. 4?2. 



9) ' The Mochiquites or Analcite Group of Igneous Eocks,' Journ. Geol., Vol. IV. 1896, 

 p. 679. 



