PLATE XLI. 



FIG. 1. 

 PHONOLITE. 



WOLF EOCK, CORNWALL. 

 Magnified 30 diameters. Ordinary light. 

 The minerals represented are nosean (27), sauidine (26), and segirine. 



The nepheline is present as a constituent of the ground-mass and can, as a 

 rule, only be recognized by the use of a high power and with a careful adjustment 

 of the light. The small green microlites are in most if not in all cases asgiriue. 

 This rock was first described by Mr. ALLPOBT. 



. 2. 

 LEUCITOPHYKE. 



EIEDEN, THE EIFEL. 



Magnified 25 diameters. Ordinary light. 

 The minerals represented are nosean (27), leucite (29), pyroxene (7). 



Nepheline is present as a constituent of the groundmass, but is not recog- 

 nizable in the figure. 



This foreign rock is introduced because leucite is an important mineral 

 which has not, as yet, been found in Britain. 



