The oil, they say, which the asphalt contains when it rises first, evaporates in the sun, of course most on the outside of the heap, leaving a tough coat of asphalt, which has, generally, no power to unite with the corresponding coat of the next mass.

Meanwhile, Mr. Manross, an American gentleman, who has written a very clever and interesting account of the lake, {150} seems to have been so far deceived by the curved and squeezed edges of these masses, that he attributes to each of them a revolving motion, and supposes that the material is continually passing from the centre to the edges, when it 'rolls under,' and rises again in the middle.

Certainly the strange stuff looks, at the first glance, as if it were behaving in this way; and certainly, also, his theory would explain the appearance of sticks and logs in the pitch.

But Messrs.

Wall and Sawkins say that they observed no such motion; nor did we: and I agree with them, that it is not very obvious to what force, or what influence, it could be attributable.