Joly — The Penological Examination of Paving-Sets. 63 



(1) Durability. 



The durability, under such conditions of wear, depends both 

 on the hardness and coherence of the rock, and on the chemical 

 stability of its .constituents ; and it may be stated at once that 

 these conditions necessitate the rejection of most of the sedi- 

 mentary rocks. These rocks, in general, fail in tho hardness of 

 their constituents or in the strength and security with which these 

 constituents are bonded together. A few which do not fail in 

 these particulars — e. g. certain quartzites and conglomerates — ■ 

 possess too uniform a hardness, as will be gathered from what 

 follows. It thus happens that paving-sets must, in most cases, 

 be selected from rocks of igneous origin. 



The Hardness of a rock involves the hardness of its constituent 

 minerals. The hardness — as tested by scratching — of the more 

 abundant rock-forming minerals is given in the following Table, 

 according to the usual scale, in which diamond is assigned the 

 numerical value 10. I further, for present purposes, classify them 

 as hard, soft, and intermediate : — 



A hard rock is necessarily made up mainly of the minerals 



whose hardness is about 6 or upwards. Of these minerals, some 



are better in the paving-set than others, as will later appear. Thus 



the frangibility of some minerals or their rapid yielding to solvent 



actions renders their hardness of little avail under the conditions 



of wear. 



G2 



