•Joly — On the Petrological Examination of Road-metal. 341 



a degree which will render it useless, or hard grains may be in- 

 sufficiently bonded or mutually attached to preserve the macadam 

 from crumbling under traffic. In short, coherence or toughness 

 is an essential quality. 



Bad wearing quality is expensive in an accelerative manner, 

 if the term may be used. For hollows in a road-surface collect 

 damp ; and this promotes further destruction in many cases. 



On the question of resistance to chemical agents, it must be 

 admitted that not much is known beyond the broad fact that 

 in any but calcareous rocks, or fragmentary rocks cemented 

 by calcareous material, physical defects generally determine the 

 destruction of the rock long before it is softened by chemical 

 changes due to surface-waters. Doubtless the impure surface- 

 water of a road appreciably accelerates the destruction of lime- 

 stone metal. It must be borne in mind that the weathering of a 

 limestone in a building or wall is not an indication of its 

 resistance in the road. This, indeed, applies to any stone. The 

 reason is twofold. The waters of the road are charged with salts 

 and organic acids ; and again, friction and crushing greatly assist 

 the solvent power of water. 



The cementitious qualities of a rock depend apparently on its 

 toughness or resistance to spalling and fracture, and on the 

 roughness of the surface of fracture and persistence of this 

 roughness under wear. Displacement upwards is due mainly to 

 the working of grit and dust into positions beneath the stone. 

 The stone must loosen, in the first instance, for this effect to come 

 about. In most cases this must be attendant on the crushing or 

 spalling of the stone upon the angles which are concerned in 

 keying it to its neighbours. It may happen that in moist places, 

 the formation of the finest dust is hindered, or remains adherent 

 to the surface of the stone, and fails to work downwards. Dust 

 and mud lodged between the stones will behave in very different 

 ways under the small movements of the stones in their beds. The 

 first will work downwards on the whole ; the second will be squeezed 

 upwards on the whole. The causes of loose working are worthy 

 of more careful study than they have as yet received. 



It will be evident that retention of a rough surface under wear 

 is not of primary importance in the same sense as it is in the case 

 of the paving-set. The surface-structure of the road is itself 



