Joly — On the Penological Examination of Road-metal. 343 



I. Felsite. 1 Geological formation, 2 Po c ^- 



Very hard, deficient in cementitious property, wears best in damp 

 or shady places. (T. A.) 



A rock of a terra-cotta colour and dull lustre : very fine-grained. 

 Only with difficulty scratched by penknife. Broken under hand- 

 lever without difficulty, suggesting that it is deficient in toughness. 



The rock is vesicular on a very minute scale, the pores being 

 hardly discernible without a strong lens. A polished surface 

 reveals to the eye a few minute but visible white felspars. After 

 five years' wear, not much rounded, but smoothed on flat surfaces. 

 The micro-section is reddish-brown in colour. 



Under microscope. — The ground-mass consists of irregularly 

 double-refracting granules, polarising in greyish colours like a 

 thoroughly devitrified glass. In this are vesicles of various sizes, 

 none large, and filled with or lined by a dirty isotropic glass, 

 containing minute indeterminate inclusions. Frequent felspars 

 occur, mostly short and stout, but some elongated ; no lamellar 

 twinning. These felspars are idiomorphic, and often show turbid 

 cavities. They polarise in grey tints : many are probably ortho- 

 clase. In some cases the felspars show a structure resembling 

 hour-glass form. Without polarised light the felspars are only 

 distinguishable by their slightly more brownish colour. There 

 is occasionally some indication of flow in their arrangement. 



Biotite in small flakes ; pale yellowish to greenish-brown ; often 

 bent; hypidiomorphic, and associated often with ragged masses of 

 red-brown oxide of iron. These masses range from dimensions 

 similar to the felspars to minutest particles, and confer upon the 

 rock its red colour. 



Remarks. — The rock is wanting in tough minerals. It will 

 crush readily, in spite of its hardness, to a fine dust. Again, the 

 fracture is too smooth to lead us to expect good binding properties. 

 Doubtless it will work less in its own dust if laid in a damp place 

 where its powder will cohere around it. 



II., III., IV., V., Acid andesites. 



Very hard, but slightly deficient in cementitious properties, but 

 wear well, especially on level stretches ; become loose on hilly roads, 

 especially in dry weather. (T. A.) 



1 Dyke 40 feet wide, running north to south. Conglomerate on either side. (T. A.) 



2 The nomenclature is that given on the Geological Survey sheets, 40, 41, 48, and 49. 



