Joly — On the Penological Examination of Road-metal. 847 



it in a fashion often seen in serpentine. Magnetite in small grains. 

 Epidote and calcite : rock is evidently considerably altered. 



IX. Geological formation, Po c i- 



A fine dark-grey, nearly black rock. Spotted with incon- 

 spicuous reddish-brown spots. Some cleavage. Under hammer, 

 breaks easily. After four and three-quarter years' wear, 

 considerably rounded and smoothed. 



Microscope : Ground-mass. — a mixture of augite (?) and very 

 small lath-felspars, apparently trielinic. Very few phenocrysts : 

 a few idiomorphic felspars with zonal structure, and with 

 simple twinning or not twinned ; rarely lamellar. A decomposed 

 mineral which appears to be olivine ; it is veined with hematite. 

 A little magnetite. Also fairly abundant spicules, green-brown to 

 opaque ; no pleochroism nor optical activity visible in these. 



In some experiments (unpublished) upon the eifect on rocks of 

 prolonged heating, alterations were obtained very similar in 

 character to the changes which appear to affect this rock. 



Remarks. — The foregoing four andesites are found to be good 

 wearing-stones. None of them are coarse-grained, and none show 

 a rough fracture. The best in wearing quality is the most acid. 

 Stone of these physical and petrological characters evidently makes 

 fairly good metal. The longest down (six years) shows no visible 

 sign of softening or decay. 



X. Olivine Dolerite, and XI. Basalt. 



Very hard and tough, splendid-ivearing stones under all conditions 

 of weather and traffic. (T. A.) 



X. Dolerite [olivine). Geological formation, d°~G n B. 



A black stone with a rough fracture, showing its crystallized 

 constituents glistening like broken sugar. No cleavage whatever. 

 Is tough under hammer. After four years' wear is considerably 

 rounded, but still with fairly rough surfaces, showing that removal 

 of angles leaves the surface rough. There is no visible sign of 

 chemical change. 



Microscope. — The rock is a typical ophitic olivine dolerite. The 

 olivine is mostly altered to serpentine. Augite abundant, ophitic with 

 lath-shaped felspars, and very fresh. This is the coarsest rock yet 

 examined. (See fig. 1, Plate XXXII.) 



