34 



THE POPULAR SCIENCE MONTHLY. 



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 prompt- 

 ly 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 re- 

 quire 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 vegeta- 



FIG. 3. A, B, PETRIFIED STUMPS NEA.B 



TWENTY MILES EAST OF THE MAMMOTH 



SPRINGS. 



tion 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 pro- 



