Page:Popular Science Monthly Volume 58.djvu/237

Rh Koch. This pavement was formed of fragments of quartz and of mastic of coal-tar, upon a bed of sandstone, the joints of which were filled with the mastic. These coal-tar streets, even with a concrete base, were not satisfactory, thus early establishing the undesirable qualities of coal tar preparations in the construction of streets.

He states his preference for the asphaltes found at Seyssel, Val de Travers and Lobsan, which are composed principally of carbonate of lime and bitumen or sandstone and bitumen. As found in nature, these asphaltes consist either of chalk, sandstone or coarser gravel which have been filled to saturation with bitumen, which when extracted or separated from the mineral constituents of the rocks, is semi-fluid, resembling mineral tar. The deposits occur in beds between more dense and barren rock, and are mined out by running galleries and tunnels

into the hills that border the valleys, in a manner similar to the mining of coal in some sections of country.

Other deposits of similar material occur at Ragussa, in Sicily, and at Limmer, in Hanover. The Seyssel and Neufchâtel rocks are generally preferred for streets, as they contain more lime and less sand, and are also freer from sulphur compounds.

On the North American continent there are deposits of vast extent both of asphaltum and asphaltes. Generally speaking, asphaltum is not used in street construction; the deposits being either too pure, and hence too valuable for such uses, or, on the other hand, so impure as to be purified only at too great cost. As the asphalte is used in enormous quantities, freight becomes a very important consideration in the selection of the material used in any given locality. This item of cost has given