The Preset valioji of I'/aii/s as Fossils. 77 



hard, woody, vascular nature stands a better chance of being 

 preserved than one of cellular structure ; while one of a 

 silicious nature, such as a diatom, has the best "expectation" 

 of all three. There are certain requisites necessary to insure 

 preservation of any plant. One of these is that it must be in 

 a position to be almost immediately covered by some material. 

 It may fall into the water, and sinking to the bottom of a lake, 

 or swamp, or morass, be covered by mud or sand. Or it may lie 

 on the sea shore and be covered by sand brought in with the 

 tide ; or finally, it may, through certain chemical properties 

 it possesses, so act upon the stone upon which it lies as to be 

 preserved, not in actual substance, but as an intaglio. 



As now generally recognized, the plants of the Coal era 

 grew upon the .shore or in the waters of swamps or marshes 

 situated upon or near the .shores of the sea, so that the tides 

 could enter, if not daily, at lea.st at intervals. O.scillations of 

 the land, an alternate rising and falling, allowed the sea 

 ingress, and then was deposited a mass of sand or gravel 

 which compressed the vegetable matter lying on the bottom, 

 and under the pressure turned it into coal. In this way was 

 preserved not only the outline, but the actual substance of the 

 plant. This, however, is not always the case. Very frequently 

 leaves occur in a fossil state which are impressions merely. 

 The sandstone shows no traces of carbonaceous matter, 

 but the outlines and the venation are as perfect as if the 

 actual leaf were before us. In these instances the particles 

 of organic matter have been replaced, particle by particle, 

 by silica. This also occurs in those cases when branches 

 or leaves are immersed in hot water, having silica in solution, 

 as in certain geysers of the Yellowstone region. Here, when 

 at times a tree .stands near enough to a geyser to receive the 

 water periodically thrown up, it will be found gradually tran.s- 

 formed into stone, suffering a living death. 



Perhaps the most remarkable method by which plants or 

 their outlines may be preserved, is by what may be termed the 

 chemical process. The possibility of this occurred to me one 

 day last Fall, as I walked along the street in the rain, and 

 looked at the fallen leaves on the pavements. I first noticed 

 numerous irregular, discolored patches on the stone slabs. 

 Looking more closely, I found that these discolorations had 

 been caused by the leaves, which had left their impress on the 



