BY W. N. BENSON. 143 



The majority of the rocks which we shall term spilites have 

 the texture characteristic of the spilites of Germany, and differ 

 from these only in their greater freshness. They are partially 

 porphyritic or glomeroporphyritic. The glomeroporphyritic types 

 have small ophitic aggregates of augite, felspar, and ilmenite, as 

 well as isolated phenocrysts of the same minerals. The ground- 

 mass varies considerably; in some rocks it has a basaltic habit, 

 but differs from basalt in the following peculiarity : the felspar 

 phenocrysts have been enlarged, their outline is no longer sharp 

 and rectilinear, but embayed, each embayment being filled with 

 a grain of augite, on either side of which there project little 

 tongue-like points of felspar (see Plate xx v., fig. 3). Somewhat 

 similar characters have been described by Rinne from the diabases 

 of the Harz Mountains(17). There is usually no flow-arrange- 

 ment of the felspars, and, though the rocks are vesicular in 

 places, there is no special alteration in texture noticeable about 

 the vesicles. 



Another type of groundmass has much greater distinction 

 between base and phenocryst, and a more marked flow-structure. 

 This is very characteristic of the zone of spilitic rocks that 

 extends north and south from Tom Tiger. 'I he sharply bounded 

 plagioclase phenocrysts have frequently microlitic extensions, 

 giving swallow-tailed, or pronged terminations. The augite 

 phenocrysts are moulded against the felspars, and do not show 

 skeletal forms. Ilmenite and magnetite form long irregular 

 aggregates of small grains. The groundmass consists of felspar, 

 in the form of strips not much shorter than the phenocryst, and 

 varying in size thence down to the finest microlitic dimensions. 

 Its outline is rarely rectilinear; it has skeletal extensions, or is 

 bent and twisted, forming shreds rather than laths. There is 

 sometimes a general flow-arrangement; at other times the felspar 

 is more radially grouped (a subvariolitic texture), or is quite 

 irregular (see Plate xxv., fig.o). Associated with the felspars, 

 are minutely granular augites and ilmenites; not infrequently 

 the augite lies in narrow streaks between the felspar shreds. 

 With decreasing ^rainsize, this texture becomes more and more 

 confused, and the rock is more readily chloritised. Skeleton- 



