and Portland cement. The details of the process of manufacture will now be given.
In the United States the manufacture of natural cements is chiefly carried on in the Lehigh Valley, near Louisville, Ky., at Akron, Ohio, at Milwaukee, Wis., and at Glens Falls and other points in the State of New York. While the rocks occurring at these different points are not identical, either in geological age or in chemical composition, they are in many respects similar. In geologic age they are of carboniferous age or older and in chemical composition they consist of limestones in which clay occurs, either uniformly disseminated throughout the rock, forming a very intimate admixture, or else interstratified with the layers of limestone, so that when the rock is broken up and burned, the resulting mixture of the constituents of the rock is very intimate. Yet intimate as the mixture is, both before and after burning and grinding, in all the ledges the rock has to be sorted and mixed in the same quarry with the greatest care, in order to insure a uniform product from the same works. From the different localities the output is sufficiently different to give the Louisville, Milwaukee and other brands distinctive, though unimportant, characteristics.
At all the natural cement works substantially the same method of manufacture is followed, although the details are modified to suit different localities.
One of the most extensive natural cement plants in the country is that of the Milwaukee Cement Co., at Milwaukee, Wis., the officers of which have kindly furnished the accompanying illustrations. Fig. 1 shows the tramway approaches to the kilns, which are arranged in a double set of ten each. The rock is quarried in the immediate neighborhood and is run in tram-cars, which are seen in the middle foreground, up the inclines to the top of the kilns into which the rock is thrown. The trestle on the right is the dump for coal, which is also loaded into tram cars, one of which is seen at the chute, and run up the incline to the kilns. The rock and fuel are thus conveniently supplied to the kilns at the top, while the burned cement is removed from the kilns at the bottom. Fig. 3 shows one of the kilns on the left; the grinding and shipping house in the center, with the inclines up which the burned cement is hauled and the railroad tracks over which the cement is shipped in all directions from Milwaukee. Fig. 3 represents the grate at the bottom of the kiln, from which the burned cement is removed, while fresh rock and fuel are supplied at the top, thus making the action of the kilns continuous.
Two obstacles make it impossible to prepare a theoretically perfect cement from the natural rock. The first is a lack of uniformity in the rock itself as it occurs in the quarry. This difficulty is obviated as nearly as is possible by careful sorting. By which the least desirable rock