J 0LY — The Penological Examination of Paving -Sets. 77 



The granite is of a grey- white colour and of uniform and 

 average coarse-grainedness. Its constituents are fresh and unaltered. 

 The felspar is for a large part microcline ; also partly orthoclase 

 and partly plagioclase. The specific gravity is 2'661. 



The felspar varies in development from idiomorphic to 

 allotriomorphic. Most often it is allotriomorphic. It sometimes 

 encloses what must be more or less spherical quartz inclusions. 

 It is free from alteration-products, or almost so. The two micas 

 are present fairly uniformly scattered over the field in large 

 hypidiomorphic crystals. The quartz is allotriomorphic, and 

 forms large areas, almost equal in size to the felspar. The 

 variation of grain of each of these constituents will be gathered 

 from the two photographs. 



As the result of many measurements the dark mica constitutes 

 about 14 or 15 per cent, of the rock. The white mica constitutes 

 about 5 per cent. There is thus about 20 per cent, of soft mineral 

 present. The quartz constitutes about one-half the remainder, and 

 the felspar the other half. 



The worn surface of the set shows abundaut and large pits 

 floored by mica and prominences of felspar and quartz, the felspar 

 standing generally a little higher than the quartz. It is evident 

 that this is due to the readier splintering and fracture of the 

 quartz, the surface of this mineral exhibiting no sign of polish, 

 but showing a broken surface or splintery roughness. The 

 felspar shows generally a fine matt surface, and in no case is 

 actually polished, as in the case of the kaolinised felspars in the 

 Caernarvon granites already described. The difference may be 

 due to the freshness and freedom from unctuous alteration-pro- 

 ducts of the Aberdeen felspar, or (less probably) to the microcline 

 structure of many of these. "We must notice, throughout these 

 observations, how remarkably uninfluential the cleavage of the 

 felspars appears to be. Quite the contrary would be anticipated 

 under the conditions of wear. 



In this set we find, then, quartz wearing with a distinctly rough 

 surface, and rather more rapidly than felspars, the fresh felspar 

 assuming a flat but still matt surface ; and both minerals, asso- 

 ciated as they are in distinct and easily deseernible grains, pre- 

 senting a surface of considerable roughness and inequality. The 

 micas, constituting about 20 per cent, of the rock, and in grains 



SCIENT. PKOC. R.D.S., VOL. X., PAKT I. H 



