Page:Popular Science Monthly Volume 43.djvu/318

 sight would seem necessary for the support of such a vigorous vegetation. It is not probable, however, that any considerable time was necessary for this purpose, for, with rare exceptions, each succeeding forest took root and began to grow very promptly after the destruction of its predecessor. In most cases the destroying flood consisted largely of mud, ashes, conglomerate, and other volcanic material, which formed an excellent base for vegetation, and it was doubtless covered with a luxuriant growth as soon as it was dried or cooled sufficiently, and this would require only a short time.

In some cases the trees grew upon a true lava base; but even then the growth began very promptly after the flow; for the upper surface of the lava soon weathered sufficiently for vegetation



to gain a footing. The growing trees then too, as at present, were frequently supported by very shallow and wide-spreading roots. We now often see large trees with such roots standing over rocks barely covered with soil; the petrified trees exhibit the same phenomena.

Besides the standing stumps, the fossil forests contain many specimens lying upon the ground. Some of these were petrified standing and then fell, and others were down before the petrifying action began. It is frequently possible to distinguish between the two by position: the first lie upon the present slope of the ground; the second often show the original surface and consequently