Joly — The Penological Examination of Paving- Sets. 81 



are present in every case; but only the soft and easily weathered 

 set will show any yielding to them. It must be remembered 

 that, like an ordinary road- surface, the conditions as regards 

 wear are those of instability. That is, the areas which become 

 depressed immediately become subject to an increased influence 

 of solvent denudation and softening : for on to these parts there 

 will be an increased gravitation of fluids, and the damp will 

 be longer retained. If, now, minerals are present readily 

 yielding to these influences, obviously the process will proceed 

 acceleratively. To the readiness with which this rock (mainly the 

 olivine) softens, I ascribe the uneven wear. The general lesson 

 may be learnt that any rock which possesses a large amount of 

 easily weathered or softened mineral — as the carbonates, zeolites, 

 mareasite and other ores, olivine, &c. — will be liable to uneven 

 wear. 



It will be apparent, from the foregoing remarks, that the prin- 

 cipal matter to attend to in the selection of the set-rock is the balance 

 between the hard and soft materials. About 75 to 80 per cent, of 

 the mineral matter present should be hard and resistant. Quartz 

 and felspar will, in nearly all cases, be the effective minerals here. 

 The felspar is not a hard mineral if alteration is very far gone. The 

 test for optical activity between crossed nicols may be considered a 

 sufficient one. The soft minerals will be mica, chlorite, and other 

 decomposition-products, olivine, and, in a less degree, augite. 

 Although I have hitherto examined no set containing plentiful 

 hornblende, it may be accepted as very probable that this sub- 

 stance will not stand among the hard minerals. The pyroxenes 

 and amphiboles may alike be classed as among the minerals not 

 durable under the conditions of wear. 



The grain of the rock must also enter into account. A suffi- 

 ciently finely divided mixture of, say, 70 per cent, hard, and 30 per 

 cent, soft, minerals might obviously afford, under wear, a surface 

 possessing its roughness on so small a scale as to be useless. The 

 soft minerals must be aggregated in grains, say, from 1 to 3 or 

 4 mm. in diameter, and the hard be in equally large grains. 

 Doubtless, considerable margin is allowable here on the ascending 



