230 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213. 
GLASS. 
Small quantities of slag have been used in Europe in the manufac- 
ture of the inferior grades of glass, but this use has never attained 
much commercial importance. In America slag has never, to the 
knowledge of the writer, been so utilized. 
ROAD METAL. 
In addition to its use as a paving material in the form of slag bricks, 
discussed in preceding sections of this paper, slag has been somewhat 
extensively used in highway construction as macadam. 
Sections of roads constructed in New Jersey with slag macadam, 
under State supervision, have proved entirely satisfactory. Near 
Buffalo, N. Y., slag has been used to some extent in highway con- 
struction, and to a greater extent in Pennsylvania and Alabama. 
The most extensive use of slag for this purpose is, however, probably 
in Maryland, where it has been utilized in highway construction in 
the counties of Baltimore, Howard, and Prince George. 
Prof. W. B. Clark refers to the use of slag for this purpose in 
Maryland in the following words:'' 
Furnace slag has been found to be, under certain conditions, a highly satisfac- 
tory road metal. It is not as valuable as the trap rocks, although its cementing 
properties are excellent, except in the case of some of the materials from the old 
furnaces. These old slags break down quickly and are readily ground into fine 
dust, and for these reasons are of little value in road construction. 
The slag from the present iron furnaces, on account of the large amount of lime 
contained in it, is very valuable as a highway material. It compacts easily when 
rolled and forms an even, smooth surface, while the fine particles unite as a hard 
cement that grows firmer with time. The iron furnaces at Sparrow Point afford 
material of this character that has already been demonstrated to be a valuable 
road metal. 
RAILROAD BALLAST. 
In several States, notably in Alabama, slag is Largely used as rail- 
road ballast. At the third annual convention of the American Rail- 
way Engineering and Maintenance-of-Way Association, a committee 
reported on this as follows: 
Blast-furnace slag containing a small excess of lime and being of a glassy nature 
will, when broken up under the track, fulfill the requirements of ballast to a very 
large degree, If a large excess of free lime be present the slag soon becomes too 
soft to hold the load, becomes concreted under the ties, and churns in wet weather. 
Slag-ballasted track usually requires a lift of 2 or 3 inches, and to be surfaced out 
of face at intervals of from two to six years, depending on the hardness of the 
slag and the amount of traffic. Slag composed almost exclusively of hard, glassy 
pieces approaches closely to the quality of rock ballast; on the other hand, slag 
with a large amount of free lime is inferior to reasonably good gravel. 
It will be noted that the noncalcareous, glass} 7 slags, which are thus 
said to be preferable for railroad ballast, are precisely the kinds that 
"Maryland Geological Survey, Vol. Ill, 1899, p. 10."). 
