82 POPULAR SCIENCE MONTHLY. 



example, ({uartz or quartzite has a hardness of seven in a scale of 

 ten, and for this quality alone it is best suited for road building 

 of any rock of common occurrence ; but it does not possess any 

 cementing power or elasticity whatever, and is therefore of little 

 use for Macadam work. As a result of long trial on roads in 

 England and on the Continent, it is found that the stone best 

 suited for road metal must possess toughness and cementing 

 qualities and as great a resistance to abrasion as is possible in a 

 stone having the first two properties. It is an important fact 

 that experimental investigation in the laboratory has pointed 

 scientifically to the same conclusions that have been obtained 

 from the severe test of long experience in actual use. 



The most important road stones are known under the common 

 names of " trap " or " dike stones." They are usually of a dark- 

 green color, are fine-grained, and are composed essentially of the 

 minerals pyroxene or hornblende and feldspar, the individual 

 minerals often not being visible to the unaided eye. Geologically, 

 they are rocks that have been forced up through fissures in the 

 earth's crust from great depths, where they existed in a melted 

 condition. Rocks of this kind are very numerous in eastern 

 Massachusetts and generally throughout the old mountain ranges 

 of the United States. The road engineer, however, has other 

 materials besides quarry stone, which, though not possessing so 

 many good qualities, nevertheless make excellent road metal 

 under proper conditions. Among these may be mentioned the 

 blue glacial gravels, kame gravels, beach pebbles, and field stones. 



Another rock in common use in various parts of New Eng- 

 land is granite (a mixture of the minerals quartz and feldspar) 

 and the allied rock, gneiss. Both these rocks are normally coarse- 

 grained, possessing a hardness, as measured by that of their com- 

 ponent minerals, a little under seven. In its use as broken stone 

 granite has certain advantages over quartz alone, in that the 

 feldspar, when pulverized or decomposed by the action of the 

 weather, has considerable cementing value ; but the decomposi- 

 tion of the feldspar liberates the quartz, and the physical difl^er- 

 ences in the matter of hardness, cleavage, etc., between the quartz 

 and the feldspar promote differential wear of the stone as well as 

 other defects. Granite, however, is an important road stone, and 

 is far superior to such rocks as limestones, slates, or marbles, 

 which, owing to their softness, are rapidly worn out. 



The production of broken stone has now assumed such im- 

 portance that several concerns in Massachusetts are making a 

 regular business of furnishing all sizes to the State or munici- 

 palities. Broken stone, as a commercial commodity, is now sold 

 on the cars at about one dollar to one dollar and seventy cents 

 per ton for the best quality of trap. 



