76 Scientific Proceedings, Royal Dublin Society. 



The photographed section (fig. 1, Plate III.) of the red granite 

 enables the grain of these granites to be compared with the rocks 

 already described, the scale of magnification being the same, 

 i.e. 12 diameters. 



The cause — botli of the durability and failure to preserve a 

 rough surface — is evidently to be traced to the too overwhelming 

 proportion of felspar. It is seen when the surface of the worn 

 set is examined that the felspar and small amount of quartz 

 compose the smooth part of the surface, and that this practically 

 occupies the entire area under wear. The very subordinate 

 depressions are shown to be due to the chloritic mineral by the 

 fact that this substance generally remains far down. These pits 

 are not numerous enougli to offer a rough surface as wear pro- 

 gresses. It is instructive to note that coarse as the rock is in 

 structure, this has not availed to preserve it from becoming polished. 

 Although slippery, almost burnished on the prominences, the 

 surface is not so generally smooth as the Penmaenmawr diorite. 

 Iron clings to the felspar and quartz eminences, and its solution 

 and oxidation has stained the surface. 



Again we may be profitably wise after the event. There is 

 little doubt that the uniform surface of felspar, unbroken by any 

 large-grained constituent differing in physical characters, has led 

 to the failure of the rock as regards retention of surface-rough- 

 ness. It is quite possible that the abundant presence of the 

 finely-divided and unctuous kaolinite throughout the felspar has 

 furthermore exerted a lubricating effect as it becomes exposed 

 under wear, much as if finely-divided black-lead was applied to 

 the surface of the set. The point seems worthy of further 

 investigation. 



The Aberdeen White Granite. 



I now come to a rock of much value for street pavement, and 

 the praises of which I have heard from several experienced city 

 engineers : theJAberdeen granite. Fig. 2, Plate III., and figs. 

 1 and 2, Plate IV., show photographs of this rock in thin 

 section. Fig. 2 is magnified to the same scale as the previous 

 photographs (twelve diameters), but figs. 1 and 2, Plate IV. only 

 four and a half diameters, fig. 1, Plate IV. being taken with 

 polarized light in order to display the large areas of almost limpid 

 microcline felspar and clear quartz. 



