Bituminous limestones are now found in many different localities. It has been discovered in all the countries of Europe, and, indeed, in nearly all parts of the world. In what manner these rocks originally became impregnated with bitumen is still a mystery which geology cannot explain. In an exceedingly interesting pamphlet published by the Montrotier Asphalt and Cement Concrete Paving Company, the following explanation is advanced :—" Gigantic masses of vegetable matter became, in the course of geological epochs, embedded in successive strata in the neighbour hood of the primitive rocks. Then, during incalculable periods, these masses were exposed to intense rays of beat penetrating from the inner fires through the crust of the earth, which had not yet grown cool. Under the action of this heat, slow combustion took place, till at length, conse quent upon a disruption of the earth's crust, a fissure, or a series of fissures, let out the imprisoned vapour, and this vapour of the consuming vegetable accumulations, escaping with a violence in proportion to the power by which it had been held back, came forth charged with the boiling bituminous substance, which it left, in passing, in the limestone strata, and the hot limestone was then impregnated with bitumen, forming this singular example of a mixture of vegetable and mineral elements." Very few of these beds of bituminous limestone have been worked; some on account of the difficulty of gaining access to them, and others on account of the presence in the rock of certain foreign substances which render the asphalt useless for industrial purposes. Of the former class, there are several mines in Spain, rich in bitumen, but placed in such inaccessible situations that they do not pay the expense of working and transporting. Of the second class, are the limestones of Auvergne, which contain so much sand and other impurities as to be quite useless. Of the mines which are worked successfully, the most important is that of Seyssel. In the neighbourhood of this town, and situate on the banks of the Rhone, there is a hill composed entirely of limestone, about 400 yards in breadth at the base. The presence of bitumen in certain portions of this hill has been known from time immemorial, as will be seen from the fact that it has been known for ages as Pyrimont, or the mount of fire. The right of working this limestone, and of extracting the bitumen, was first granted by the French Government to a man named Secretan, in the fifth year of the Republic. Secretan established a factory on the banks of the Rhone for the manufacture of asphaltic mastic, which for some years produced only small quantities, until, in the year 1838, the establishment in Paris of the first asphalt pavements gave an unexpected impetus to the manu facture, and from that time it rose to a position of importance which it has maintained ever since. In the year 1855, the output from this mine was 1500 tons ; in 1863 it reached the large amount of 10,000 tons. As regards production, this mine is still the most important, sending annually into the market, either in the rough state or in the form of mastic, from 9000 to 10,000 tons of asphalt. The factory close by the mine transforms annually from 7000 to 8000 tons of the mineral into mastic. The chief characteristics of the Seyssel limestone is the extreme variableness of the appearance of the mineral, and of its richness in bitumen, the constituents, pure limestone and bitumen, remaining always the same.
The Val-de-Travers mine is very different from that of Seyssel. It is much richer in bitumen, but of considerably less extent. The bed of asphalt is covered with a thin layer of soil, underneath which is another layer of earthy asphalt, varying in thickness from 2 ft. 6 in. to 3 ft. The bed itself is circular in form, about 22 ft. thick and 160 yards in diameter. It contains 12 or 13 per cent. of bitumen, and it was the first kind ever employed in the construction of pavements.
These two mines are by far the most important sources of asphalt ; but there are several smaller ones from which an equally good product is obtained. Among these are those of Chal louge, Chavaroche, Manosque, Lobsann, Ballet, and Pont du Chateau, all of which are still being worked profitably. There is also a large mine at Maestu, near Vittoria, in Spain, the product of which is of a very fine quality. Unfortunately, access to this mine eau only be gained by means of mules and oxen, which is a serious drawback to its successful working.
The processes by which the rock is prepared for the market are four in number, viz.: (1) Extrac tion from the mine ; (2) Preparation (powdering, sifting, &c.) ; (3) Boiling, and (4) Casting into blockfi. The details of these processes are as follows :— Extraction.—Asphalt is ordinarily obtained from the mine by blasting, like other rocks. This is sometimes carried on iu the open air, as at Seyssel and Val-de-Travers, and sometimes in underground workings, as at Challenge and Chavaroehe. In winter, owing to the hardness of the rock, the work is much easier than in summer, when it is more or less soft and sticky. It happens sometimes that the elasticity of the mineral cannot be overcome by gunpowder, in which ease it must be hewn out with the pick. In the very hot weather, the miners work for only a few hours in the morning, before the rock has had time to soften under the influence of the RID These remarks do not, of course, apply to the extraction of the rock from underground workings. whero these obstacles are avoided by the unvarying low temperature of such workings. The blocks of ruineral should never be piled up in high heaps, as in such a case, an elevation of tempersture would cause the undermost blocks to crumble to pieces, when, should the fragments become mixed with rain-water, the subsequent operations are much impeded.
Preparation.—The preparation of the mineral consists in breaking it up, reducing it to powder, and passing it through sieves. The blocks of asphalt aro generally reduced to the required size by passing them through a species of crushing-mill, constructed of two large metal rollers, placed at any required distance from each other, and furnished with steel teeth. During the summer, the asphalt is mostly broken by hand, the operation being much impeded by the softness of the rock, which almost resemblee lead in consistence. The cost of breaking varies in winter and summer in the proportion of 3 to 5.
The pulverization of the broken asphalt is effected in one of two ways, each of which admits of two different processes. If it be powdered by decrepitation, the rock may either be heated by means of a stove or by means of steam. If it be reduced to powder by crushing, either metal rollers or an ordinary crushing-mill may be employed.
The first of these methods, viz. decrepitation, is now practically abandoned, owing to ita high cost, as compared with the other. It will not therefore be necessary to describe the two processes here.
The second of these methods, that of crushing the asphalt, is performed by means of two pairs of metal cylinders, one of which serves to break the stone roughly, and the other to complete tho pulverization. This method is always employed with the more homogeneous rocks, which do not enclose particles of ordinary limestone. The cylinders of the first set are armed with epikes about 5 or 6 centimetres in length, which overlap ono another ; they are placed at such a distance from each other, that the rock is broken to about the size of an egg. After passing between these rollers, the crushed roek is passed by means of a hopper through the second pair. These are perfectly smooth, and placed only about half an inch apart ; their effect is to crush the fragments into a thick cake or paste, in whioh state the asphalt is ready for conversion into mastic.