342 Scientific Proceedings, Royal Dublin Society. 



sufficient to afford the requisite bite to the horse-shoe. In a 

 secondary way, however, a rough-fracturing stone appears to be 

 best for road-metal. The cementitious quality is undoubtedly in 

 part dependent on the frictional hold of one stone on another, 

 and on the packing. 



It appears then that, in a road-metal, we should look for 

 coherence and toughness along with a rough surface of fracture. 

 There is some mechanical advantage in a heavy stone, but a dis- 

 advantage on the score of expense. A stone giving a white road 

 is doubtless, other things being equal, the best. First cost has, 

 of course, to be considered in every case. 



Of the sedimentary rocks, quartzite makes an excellent macadam 

 if it is really coherent. Limestone makes a metal which rapidly 

 crushes and turns to a very slippery mud. Among the igneous 

 rocks are many of great value. Granites are often badly co- 

 herent, and soon wear into freestone. They, in many cases, lack 

 toughness. The group of felspar- augite rocks, perhaps, make the 

 best macadam of any. Of these the dolerites have roughness of 

 grain and great toughness. This toughness is often dependent on 

 the " ophitic structure " of these rocks. In this type of structure, 

 the augite acts as a matrix in which the felspar is imbedded. It is, 

 however, of primary importance to make sure that the augite is 

 not weakened or softened by decomposition. The veining of this 

 mineral with decomposition-products on a very minute scale is 

 apparently sufficient to determine the weakness of the stone. 

 The augite is often completely altered to serpentine or to chloritic 

 substances. Such rocks are greatly inferior to dolerites, having 

 a fresh augite. This point is very clearly shown in the practical 

 and analytic notes which follow. 



A basalt of the coarser sort also may be a capital road-metal. 

 And there is a long category of andesites, both the more acid and 

 more basic, which make more or less good macadam. The andesites, 

 however, suffer apparently from their too smooth fracture-surfaces. 

 Samples of andesites after some years of wear, which Mr. Aitken 

 kindly forwarded to me, show a smoothness which might almost 

 rival the frictional-solution surface of a water-worn stone. 



In what follows the practical notes are in italics. In the case 

 of the first sixteen specimens, these notes are by Mr. T. Aitken, 

 and are given verbatim and lettered (T. A.). 



