Joly — The Penological Examination of Paving-Sets. 69 



the set to a depth of one or two centimetres ; and by their action 

 on the more yielding minerals affect the alteration of the less 

 stahle substances : in fact, reversing the usual order, and antici- 

 pating the mechanical effects. 



In brief, durability depends on the hardness, coherence, bond- 

 ing, and chemical stability of the crystallized grains of which the 

 rock is composed. The hardest common substances are quartz 

 and felspar. The resistance to wear, in general, varies directly 

 as the amounts of these minerals present. The sufficient bonding 

 of the minerals is generally attained in the holocrystalline igneous 

 rocks. But the rock must in all cases be fairly fresh. It follows 

 that the granites, syenites, diorites, gabbros, dolerites, and some 

 basalts may make good sets, the durability diminishing (in a 

 general way) along with the silica percentage of the rock. 

 Porphyritic and vesicular rocks are to be avoided, if these 

 characters are at all marked. 



(2) Surface Koughness under Wear. 



We now come to consider the more difficult question as to 

 what features of the rock will enable us to pronounce on its 

 probable fulfilment of the second condition : that of retaining a 

 rough surface under wear. 



In order that the set, exposed to friction, should remain rough, 

 some of its constituents must yield before others : those that resist 

 longest always standing above the general surface, and so giving 

 a coarse file-like or granular formation upon which the horse- 

 shoe will bite. Durability further demands that the outstanding 

 grains should be of hard and strong material. Thus, in granites, 

 the mica goes first ; and the felspars and quartz remain to act as 

 the teeth of the file. Minute and careful examination of the 

 worn surface of the set by aid of a strong lens generally shows 

 clearly enough which mineral constituents have yielded, and 

 which have held out. In this examination careful washing of 

 the worn surface, using a stiff brush, is needful. It must be 

 carried on in a good light : direct sunlight is the best. 



From these remarks it will be inferred, in the first place, 

 that a certain coarseness of grain is desirable. If the grain of 

 the rock is too fine, the result under wear is a surface the- 



