POSSIL FORESTS OF YELLOWSTONE PARK. 



19 



which was doubtless then, as it is to some extent now, a marked fea- 

 ture of the park region. At least a hint of the probable process is 

 afforded by the action now going on in the hot spring areas. Many 

 of those areas are closely surrounded by forests, and unless the action 

 of the springs is very violent the trees may be growing only a short 

 distance away. Occasionally a hot spring may break out near the 

 edge of a forest, the first effect being, of course, to kill the trees. 

 In a few years, by the action of the ordinary processes of decay, a 

 tree so killed may have lost its bark and most of its smaller branches. 

 The hot water which constantly or intermittently surrounds the 

 tree contains a considerable amount of silica in solution, and as this 

 hot silica-charged water is drawn up into the wood by capillarity the 

 silica may be deposited in the cells of the wood after the water cools 

 or evaporates. The first result will be a more or less complete cast 

 of the interior of the cells and vessels of the wood. This much of 

 the process has actually been ob erved, but as decay is more rapid 

 than silicification, the wood crumbles to dust before petrifaction is 

 complete. If the trunk could be surrounded by ashes or mud and 

 thus protected from atmospheric action, it might in time be com- 

 pletely turned to stone. 



The fossil forests are surrounded by a matrix that is known as an 

 acidic lava — that is, a siliceous lava — which contains abundant silica 

 in solution. The first part of the process of silicification may well 

 have been that above described as taking place in the hot spring areas 

 at the present day — that is, the silica would be deposited in all the 

 cells and vessels of the wood, making an accurate cast of all open 

 spaces. Then, while the slow process of decay went on, as each par- 

 ticle of organic matter was removed its place was taken by the silica, 

 until, finally, all the wood substance had disappeared and its place 

 atom by atom had been taken by silica. 



By this or a similar process the wood has been preserved or fossil- 

 ized with remarkable fidelity; in fact, thin sections or slices of the 

 fossil wood may be studied under the higher powers of the micro- 

 scope with almost or quite as much completeness and satisfaction as 

 if they were sections cut from a piece of living wood. Each cell and 

 vessel, with its characteristic pits and markings, is preserved exactly 

 as it grew. Some of the wood, however, was evidently more or less 

 decayed before it was fossilized, or else decay worked faster than 

 replacement, so that in some fragments the structure is not so clearly 

 preserved. Many of the tnmks were subjected to pressure before 

 replacement was complete, and this has crushed or distorted the cells. 

 On the whole, however, the wood is exceptionally well preserved, as 

 may be seen in figures 12, 13, 14, and 15. These are all magnified 

 100 diameters and were photographed directly from the thin sec- 



