Page:Popular Science Monthly Volume 81.djvu/28

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The surface of the deposit or lake is not a uniform expanse of pitch. It is grassy along the edges and becomes free from vegetation at some distance from the center. Shrubs and small trees occur in a few cases known as islands. These patches move from place to place with the movement of the pitch at the surface. The main mass of asphalt is a broad expanse of pitch made up of separate areas of irregular outline, but at times quite circular, which are separated by channels, filled with rain water, which prevents their coalescence. The boundaries are depressed and the center of the areas is always somewhat elevated above the edges, that is to say, they are mushroom-like. The origin of the separate areas evidently lies in the constant movement of the crude material, due