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Y. 



THE PETEOLOGICAL EXAMINATION OF PAVING- SETS. 

 Part I. By J. JOLY, B.A.I., Sc.D., F.B.S., F.G.S., Professor 

 of Geology and Mineralogy in the University of Dublin, Hon. Sec. 

 Eoyal Dublin Society. 



(Plates II.-V.) 



[Read March 17 ; Received for Publication, March 27 ; Published July 23, 1903.] 



An ideally good paving-set should fulfil the following condi- 

 tions : — 



(1). It should be durable. 



(2). Retain a rough surface in wet and dry weather. 



(3). Wear down uniformly. 1 



The conditions of wear to which a set is exposed are of the 

 severest. The stone has to resist endless hammering : not only the 

 fast hoof-stroke of the lightly-laden horse, but the slow and sledge- 

 hammer footfall of heavy draught-horses, as well as the shocks from 

 the iron wheel-tires, the blows from which must often be of extreme 

 violence. And this hammering, rubbing, and crushing is more 

 often than not applied in presence of the impure surface-waters. 

 Mechanical forces are here applied in the most destructive form ; 

 while the conditions of solution and alteration are those which 

 Daubree long ago showed to be the most effective : i. e. attrition in 

 presence of a solvent. 2 



My object in these notes is to inquire if petrological examina- 

 tion of the rock may not suffice to determine beforehand how far 

 a particular stone possesses the qualities necessary to satisfy the 

 conditions indicated above. 



1 Of course other practical economic considerations enter, suoh as ease of dressing, 

 and cost of carriage. In the last item the specific gravity of the rock enters as a factor. 



2 Geologie Experimentale, I., p. 208. 



