Note 8 — Chapter IV · THE SELECTION OF MATERIALS FOR MACADAM ROADS[7]
Reading edition
This term is derived from the Swedish word trappa, meaning steps, and was originally applied to the crystallized basalts of the coast of Sweden, which much resemble steps in appearance. As now used by road builders, it embraces a large variety of igneous rocks, chiefly those of fine crystalline structure and of dark-blue, gray, and green colors. They are generally diabases, diorites, trachytes, and basalts.—Page.