PLATE XXXIX. 



FIG. 1. 

 SPHEKULITIC FELSITE 



ARRAN. 



Magnified 10 diameters. Ordinary light. 



In this rock there are no definitely recognizable minerals. The darker 

 portions of the figure are crowded with ill-defined, brownish granules. The 

 lighter portions are crystalline and often give definite extinction over considerable 

 areas. The distribution of the brown granular matter has been determined by 

 the crystallization of the substance which forms the lighter portions. 



In the irregular patch (A), somewhat to the left of the centre, there is no radial 

 structure. The lighter spots extinguish as individuals. In the other portions 

 of the figure a radial structure may be seen. This is due to the crowding 

 together of the brown granular matter along certain lines. When the section 

 passes through the centre of a spherulite a nucleus, in which there is no radial 

 structure, is sometimes seen (B) and this nucleus is surrounded by two zones 

 showing radial structure. When the section is not cut through the centre of the 

 spherulite the ends of the radiating bundles are seen. Where several spherulites 

 have been developed in close juxta-position the mass is divided into polygonal 

 spaces. 



The radial structure is not so well seen under crossed nicols as it is in 

 ordinary light. The spherulites do not give a black cross, but are composed of 

 irregular crystalline patches which give uniform extinction. (Compare structure 

 developed in glass by devitrification. HERMAN & BUTUSY, Proc. Koy. Soc., 1885, 

 p. 87). This rock has been described by Mr. ALLPORT (Gr.M., 1872, p. 536), and 

 Professor BONNEY (G.M., 1877, p. 499). 



FIG. 2. 

 AUGITE-GBANITE. 



LINHOPE SPOUT, CHEVIOT. 



Magnified 25 diameters. Polarizer only. 



The minerals represented are augite (7), dark-mica (9), quartz (24), felspar 

 (10) and magnetite. 



The augite is pale in colour and belongs to the rnalacolite variety. The dark- 

 mica is strongly pleochroic. Magnetite occurs as inclusions in the augite and 

 dark-mica. The felspar is partly striated and partly unstriated. In other parts 

 of the same slide the quartz and felspar form micro-pegmatite. 



