Joly — The Penological Examination of Paving- Sets. 71 



these soft areas must be distributed with fair uniformity through- 

 out the rock. Finally, the total percentage of the area occupied 

 by soft miuerals must not exceed a certain amount, if durability 

 is to be secured. This limit may be about 25 or 30 per cent. In 

 certain granites a less percentage is found consistent with the 

 necessary surface- roughness. Other circumstances of rock struc- 

 ture in some cases enter the question as to whether the rock will 

 make a completely satisfactory set or not. 



Examination of some Typical Sets. 

 The Penmaenmawr Enstatite Diorite. 



Before going further into the matter, it will conduce to clear- 

 ness if some typical sets are more closely examined. 



The first I take is the blue Penmaenmawr set already 

 referred to. The rock is well known. Some varieties are coarser 

 grained than others. The one I am now dealing with is from the 

 finer-grained rock, and was used in Dublin. It is labelled by 

 the engineer, " In a very slippery condition." 



This rock breaks with a clean conchoidal fracture, sharp on 

 the edge (always an indication of fine-grainedness). To the 

 unaided eye, it shows no individual grains. Its colour is a dark 

 greenish-blue. It is an enstatite diorite, or enstatite diabase, and 

 has been described by Phillips (Q. J. Gr. S., vol. xxxiii. (1877), 

 p. 423), and more recently by Teall (" British Petrography," 

 pp. 272 et seq.). The rock is rather above the average weight: 

 specific gravity, 2-827. 



The section given in fig. 1, Plate II., is a photograph by 

 •ordinary light, magnified 12 diameters. The slice used is from a 

 chip of one of the sets. 



The coloured constituents are a very pale-brown augite in a 

 fresh condition, showing little or no pleochroism ; often twinned : 

 and, considerably more abundant, and about the same in the size 

 of the individual crystals — a fibrous rhombic pyroxene, extinguish- 

 ing longitudinally. The colour of this mineral is very pale-brown, 

 and the pleochroism faint : green in the direction of the principal 

 axis: probably an enstatite rich in iron. Biotite is also present 

 in irregular plates which are occasionally chloritized. This is 

 much less abundant than either of the other coloured constituents. 



