PLATE XXIII. 



FIG. I. 

 PORPHYEITIC BASALT. 



THE LION'S HAUNCH. ARTHUR'S SEAT, EDINBURGH. 



Magnified 25 diameters. 



The porphyritic constituents are plagioclase (10), augite (7), and olivine (1). The 

 constituents of the ground-mass are plagioclase, magnetite, augite grains and crystals. 



A section of augite cut somewhat oblique to the vertical axis occupies the upper 

 part of the figure to the left. On the right is a portion of one of the porphyritic 

 felspars. It contains large irregular inclusions of the ground-mass in the centre, and a 

 zone of smaller inclusions round the margin. The felspars of the ground-mass often 

 occur as microlites, with ragged or bifid terminations. Near the centre of the figure is 

 a rectangular section of a skeleton felspar, with the interior occupied by the ground- 

 mass of the rock. The augite microlites are too small to be distinctly represented in the 

 figure. Isotropic glass, if present at all, must be so only in extremely small quantity. 



The olivines are altered to serpentine along the cracks. This figure has appeared 

 in " Cole's Studies in Microscopical Science.'' 



FIG. II. 

 DOLERITE. 



HAILSTONE HILL, ROWLEY, STAFFORDSHIRE. 



Magnified 20 diameters. 



The minerals represented are plagioclase (10), augite (7), and titaniferous iron 

 ores. 



The specimen from which the slide was taken is a coarse-grained portion of the 

 dolerite represented in fig. 2, plate XI. The augite and felspar are intergrown exactly 

 in the same manner as quartz and felspar in pegmatite and micro-pegmatite. Each 

 mineral depolarises uniformly over large areas. The structure is due to the simultaneous 

 crystallisation of felspar and augite, and differs therefore from the ophitic structure 

 which is determined by successive crystallisation. 



This rock is described by Mr. ALLPORT (Q.J.G.S., vol. XXX., p. 549), and the 

 figure is drawn from his original slide. 



