ELISHA MITCHELL SCIENTIFIC SOCIETY. 69 



]ieav\-. it will sometimes be found better and more 

 e<:onomical to obtain a superior material, even at a higher 

 cost, than the local stone; and in cases where the traffic is 

 verv oTeat the best material that can be obtained is the 

 most economical.""^ In the middle and western counties 

 of the State, in many places, stones now covering the cul- 

 tivated fields will be found satisfactory for use on the roads, 

 and in order to get rid of them farmers will haul and sell 

 them for low prices. 



Stones ordinarily used in the construction of ^Macadam 

 and Telford roads are the following: Trap, syenite, granite, 

 gneiss, limestone, quartzite, gravel and sand. The first 

 three of these names are used here in a very general sense, 

 and include several species of rock which, in technical 

 language, would be known by other names. In general, 

 it mav be said that they rank in importance about in the 

 order named, but several of them, especially the granite, 

 gneiss and limestone, vary so much in quality that this 

 general statement is subject to modification accordingly. 



The term trap, as here used, includes not only the black, 

 rather fine-grained, igneous rock known as diabase, which 

 occurs in long dykes in the sandstone basins of Deep and 

 Dan Rivers, but also the somewhat similar material which 

 is to be found in the older crystalline rock of many other 

 reo^ions. In this State it is often known local Iv under the 

 name of ''nigger-head'' rock. This rock does not usually 

 split well into paving blocks, but when properly broken it 

 is the most uniformly good material obtainable for macadam- 

 izing public highways, though sometimes it does not 

 ''bind'' well. 



Syenite, sometimes called Jwvjiblende granite, varies 

 somewhat in quality and composition. It is a widely dis- 

 tributed rock in the midland and western counties of 



=^Byrue, Highway Construction, p. 



