74 Scientific Proceedings, Royal Dublin Society. 



twinning. A little apatite is present, associated with the 

 decomposition-products of the mica. 



The felspar and quartz are in grains of almost equal dimensions, 

 interfiled most generally on irregular boundaries. The grain is 

 fine for a granite. Quartz constitutes perhaps 20 to 25 per cent. 

 of the rock : the softer minerals hardly 10 per cent. ; the 

 remainder being felspar. The comparatively fine-grained relation 

 of felspar and quartz is important, when considered along with 

 the small percentage of soft minerals. 



Examination of the photographed section (Plate II., fig. 2), 

 and comparison with some given later (Plates III. and IV.) of 

 the excellent Aberdeen granite, will best show how fine-grained 

 is the Caernarvon rock : the Aberdeen being a granite of only 

 average grain. 



Surface-examination of the worn set shows that the fine-grained 

 felspar and quartz are the outstanding part of the rock. The areas 

 of felspar and quartz wear into flat-topped eminences, the chlorite 

 and remaining mica come away. A little mica can be seen in the 

 bottoms of the hollows. The slipperiness, such as it is, is mainly 

 due to the fact that by far the greater part of the rock, certainly 

 over 90 per cent., is composed of hard and resistant minerals which 

 wear into smooth surfaces without sufficient intervening gaps to 

 give an altogether satisfactory bite. The pits are indeed fairly 

 coarse, but too few in number. That considerable bite exists is 

 evidenced in the scraped-off iron adhering strongly to the felspar 

 and quartz. Again, in this case, the large amounts of felspar, 

 along with the considerable amount of quartz, might have enabled 

 us to predict, from the microscopic examination of the rock, that 

 the set would be durable under wear. 



The minute intermixture of felspar and quartz, that is, the 

 fine-grainedness of the hard constituents, taken in conjunction 

 with the small percentage of soft minerals, might also have led to 

 the inference that a smooth surface would develop, composed of 

 the finely-intermixed, hard minerals, insufficiently broken up by 

 the removal of the chloritic mineral. 



It is remarkable that the impure surface-waters of the street 

 have penetrated into the set, and discoloured it to a depth of over 

 a centimetre from the surface and top sides. The rock becomes 

 brownish in colour under this influence ; and the soft green mineral 



