OF THE DEPENDENT ISLES OF TAIWAN. 23 



is the case of glass-enclosures, save that ground-mass which fills 

 up the rectangular space between the lamellae, showing as if the 

 larger crystals have grown out by the apposition of numerous 

 flowing lamellae. Penck^^ holds the same view, as given here. 

 In consequence of this lamellar composition, which is one of 

 the causes of the paucity of inclusions, both terminations of the 

 ledges became indented and forked, after the manner of a 

 parapet {PL I, Figs. 4 and 6., PI. II, Fig. o), a characteristic com- 

 mon to all the plagioclases of Basalts. It seems more rea- 

 sonable to consider these monstrosities as incipient forms of groicth, 

 having simultaneously many centres of crystallisation in space, 

 which in later stages have groivn together to make up one in- 

 dividual unth but internal complex compositions. Morphological 

 and optical homogeneities are, however, frequently disturbed 

 through the flowing motion and sudden cooling of the consolidat- 

 ing magma. Several stages of similar kind in crystallisation 

 may be frequently observed under the microscope in the forma- 

 tion of artificial crystals. ' 



Symmetrical but contrary extinction takes place at the 

 maximum angle of 33° — 3ö°, with reference to the suture of the 

 albite-twinning, and the extinction with regard to the pericline- 

 lamellae amounts to —16°, showing that the plagioclase is of a 

 basic labradorite. It is easily acted on by HCl. The Baveno 

 twins were once observed. 



The plagioclase in the ground-mass is lath-shaped, extremely 

 slender, and polysynthetic ; termination being also a parapet-like. 

 The habit of crystals is prismatic, and such a form is said to 

 be elongated parallel to the a-axis. This is indeed true ; for, 



1) ' Studien ueber lockere vulcanische Answüröinge.' Zeit-fhr. d. d. geol. Gesdl. Bd. 

 XXX., S. 101. Taf. v., Figs. 3, 5, and 7. 



