Page:Popular Science Monthly Volume 81.djvu/30

24 flaked out with picks, in large conchoidal masses weighing from 50 to 75 pounds. It is honeycombed with gas cavities, and resembles in appearance the structure of a Swiss cheese. The gas evolved by the pitch consists of hydrogen sulphide and carbon dioxide, but in such proportions that it burns readily when a match is applied, although the flame is not maintained for any length of time.

In the center of the so-called lake is a point where there is a continued influx of soft material, accompanied by a stronger evolution of gas, which gradually hardens and becomes like the remainder of the deposit. The point of evolution of the soft material moves about from

place to place along lines of least resistance. As that evolved at one point hardens the fresh material breaks out elsewhere. It is peculiar in that it is associated with so much free water which rises with it, that it can be handled freely and made into a ball without adhering to the hands. It is in an active state of change, since if it is sealed in a tin can the gas evolved will, in a few weeks, burst the containing vessel.

The temperature of the pitch at the surface is no greater than that of the surrounding air except when it is exposed to the noon-day sun, when it may rise to 130° F. or over. That of the soft pitch is no higher than any other part of the deposit.