The term land or overflow pitch is applied to the deposits of asphaltum found outside the lake. These deposits form extensive beds of variable thickness, and are covered with from a few to several feet of earth; they are considered by some authorities to be formed from pitch which has overflowed from the lake; by others to be of entirely different origin. The name cheese pitch is given to such portions of the land pitch as more nearly resemble that obtained from the lake.
The term iron pitch is used to designate large and isolated masses of extremely hard asphaltum found both within and without the bor ders of the lake. It is supposed to have been formed by the action of heat caused by forest fires, which, sweeping over the softer pitch, re moved its more volatile constituents. .
The name epvree is given to asphaltum refined on the island of Trinidad. The process is conducted in a very crude manner, in large, open, cast-iron sugar boilers.
The characteristics of crude Trinidad asphaltum, both lake and land, are as follows: It is composed of bitumen mixed with fine sand, clay, and vegetable matter. Its specific gravity varies according to the impurities present, but is usually about 1.28. Its color, when fleshly excavated, is a brown, which changes to black on exposure to the at mosphere. When freshly broken; it emits the usual bituminous odor. It is porous, containing gas cavities, and in consistency resembles cheese. If left long enough in the sun, the surface will soften and melt,
and will finally flow into a more or less compact mass.
Refined Trinidad Asphaltum. The crude asphaltum is refined or purified by melting it in iron kettles or stills by the application of indirect heat.
The operation of refining proceeds as follows: During the heat ing, the water and lighter oils are the asphaltum is lique fied; the vegetable matter rises to the surface, and is skimmed off; the earthy and siliceDus matters settle to the bottom; and the liquid asphal tum is drawn off into old cement or flour barrels.
When the asphaltum is refined without agitation, the residue remaining in the still forms a considerable percentage of the crude material, frequently amounting to 12 per cent; and it was at one time considered that the greater the amount of this residue the better the quality of the refined asphaltum. Since agitation has been adopted, however, the greater part of the earthy and siliceous matters is retained in suspension; and it has come to be considered just as desirable for a part of the surface mixture as the sand which is subsequently added. The refined asphaltum, if for local use, is generally converted into cement in the same still in which it was refined.
The average composition of both the land and lake varieties is shown by the following analyses: • Average Composition of Trinidad Asphaltum