Page:Popular Science Monthly Volume 81.djvu/29

Rh to the evolution of gas at the center, from which point the pitch rolls over toward the edges. This is shown by the fact that pieces of wood which emerge erect at the center are gradually carried to the circumference, their deflection from the perpendicular increasing as the distance from the center increases. At the channel they topple over and are again engulfed in the pitch. This illustrates very well the activity of the entire surface of the deposit, although it is much more active near the center of the lake.

As to the depth of the lake, borings made in 1894 at the center, were carried to a depth of 135 feet, by means of a wash drill, the entire distance being through asphalt of the same character as that at the surface. This result shows the great depth of the crater, and the uniformity

of the material which it contains. At the pitch lake, therefore, at a point 138 feet above the sea, there is a bowl-like depression, more than two thousand feet across, and over 135 feet deep, reaching to the sea level, and filled with a uniform mass of asphalt, a mass which must amount to many million of tons, making it the largest deposit of solid native bitumen in the world.

The material forming this deposit is an emulsion of water, gas, bitumen and mineral matter, the latter consisting largely of fine sand and a lesser amount of clay. It is in constant motion, owing to the evolution of gas, and for this reason whenever a hole is dug in the surface, whether deep or shallow, it rapidly fills up, and the surface resumes its original level after a short time. While sufficiently soft to accommodate itself to any change of level and to slow movement, it can be readily