J 0LY — On the Penological Examination of Road-metal. 345 



twinning in lath-shaped felspars. Phenocrysts absent. Some 

 chloritic areas occur. No glass. The structure might be described 

 as pilotaxitic. In this ground-mass occur (a) a greenish, fibrous 

 mineral, same size as felspars, idiomorphic; polarises golden 

 yellow ; extinction not quite straight ; pleochroic green to colour- 

 less : cegerine ?, (b) magnetite in grains, dust, and crystals, and a 

 little hematite. 



V. Geological formation, Po c i- 



Very fine-grained, black rock. Fracture conchoidal. Hard, 

 but possesses some cleavage. 



Microscope. — Same generally as IY. An interstitial substance 

 appears, however, between felspars making up the ground-mass. 

 This is occasionally spherulitic, and is associated with a strongly 

 refracting colourless substance, not clearly outlined, but occasionally 

 showing traces of prismatic form, and then cracked transversely. 

 Felspars in ground-mass are very distinct, clean, and sharp. 



Remarks. — Of these acid andesites, II., III., Y. are not 

 conspicuously tough rocks. They are also fine-grained, and 

 not rough on fracture. IY. is very tough, but is of fine grain. 

 The last should not fail by crumbling ; but its comparatively 

 smooth fracture is doubtless the cause of its faults. The report 

 of their qualities is, on the whole, favourable. They are examples 

 of fine-grained, fairly tough rocks, wearing well, but failing in 

 some degree, owing probably to possessing too smooth a surface of 

 fracture. 



VI., VII., VIII., and IX. Andesites. VI. is an acid variety, 

 the others basic. 



Sard and fairly tough, good-wearing stones. iVb. VI. best. 

 (T.A.) 



VI. Geological formation, Po c i- 



This is a nearly black, fine-grained rock. In it the phenocrysts 

 are easily seen by eye by reflected light. Some of these crystals 

 are up to 2 or 3 mms. in length. The greater part of the rock 

 is fine, and not resolvable even with a lens. Specimens taken up 

 after six years' wear are much rounded and surfaces smoothed. 

 Under the hammer it breaks fairly easily, showing no cleavage. 



