70 Scientific Proceedings, Royal Dublin Society. 



roughness of which is on too minute a scale to afford bite, especi- 

 ally in presence of fine mud ; and sanding, whereby hard quartz 

 particles are supplied to act as temporary teeth, becomes necessary 

 — a resort which, perhaps, ultimately intensifies the evil it is 

 intended to remedy : the sand undoubtedly acting as a polishing 

 and abraiding agent. It is on account of its fineness of graiu 

 that the well-known Penmaenmawr rock, or Welsh blue set, fails 

 to preserve a sufficiently rough surface. Such sets possess a 

 durability inversely as their surface-roughness. Obviously the 

 set, once worn smooth, escapes to a considerable extent further 

 wear. A. fine-grained variety of the Welsh Penmaenmawr set 

 just referred to was labelled by the engineer — to whose kindness 

 I owe most of the other sets — " In a very slippery condition." 

 Rocks so fine-grained as this, quite grainless to the unaided eye, 

 should be avoided where a rough surface is desirable. For use 

 on inclines, they should not be thought of. 



While a certain coarseness of grain is desirable, on the 

 other hand, a too coarse grain may defeat the end in view. 

 Thus granites possessing large continuous developments of fel- 

 spar may wear to a surface of considerable slipperiness. The 

 hard areas here stand up, and becoming flat on top, refuse to 

 allow the shoe to bite on the minor inequalities due to the destruc- 

 tion of the soft constituents. This is a common evil. Both in 

 wet and dry weather, sets of this character will be more or less 

 slippery, although, perhaps, never so dangerous as the set which 

 owes its slippery quality to general polishing down of the whole 

 surface : probably because a coarse sort of inequality continues. 

 In the case of the fine-grained set, what bite there is after the 

 smooth surface is attained must be due largely to the joints 

 between the sets. More especially is this true in muddy streets. 

 What degree of coarseness of structure is desirable ? Only 

 examination of successful and unsuccessful paving-sets can enable 

 us to answer this question. Such examination will be entered on 

 presently, and the subject gradually developed. Many modifying 

 circumstances, not easily specified in a few words, must enter any 

 general rules laid down. It may be stated that the softer 

 minerals, whether as single crystals or aggregates, should expose 

 on the surface of the set areas above a certain dimension, in order 

 that sufficient pitting may occur, and that, for uniformity of wear, 



