74 THE PRESERVATION OF PLANTS AS FOSSILS. [CH. 



end of the specimen, the woody tissue has been partially pre- 

 served by the infiltration of a solution containing carbonate 

 of lime (fig. 12, 2). 



Numerous instances have been recorded from rocks of 

 various geological ages of casts of stems standing erect and 

 at right angles to the bedding of the surrounding rock. These 

 vertical trees occasionally attain a considerable length, and 

 have been formed by the filling in by sand or mud of a pipe left 

 by the decay of the stem. It is frequently a matter of some diffi- 

 culty to decide how far such fossils are in the position of growth 

 of the tree, or whether they are merely casts of drifted stems, 

 which happen to have been deposited in an erect position. The 

 weighting of floating trees by stones held in the roots, added 

 to the greater density of the root wood, has no doubt often been 

 the cause of this vertical position. In attempting to determine 

 if an erect cast is in the original place of growth of the tree, it is 

 important to bear in mind the great length of time that wood is 

 able to resist decay, especially under water. The wonderful 

 state of preservation of old piles found in the bed of a river, 

 and the preservation of wooden portions of anchors of which the 

 iron has been completely removed by disintegration, illustrate 

 this power of resistance. In this connection, the following 

 passage from Lyell's travels in America is of interest. In 

 describing the site of an old forest, he writes^ : 



"Some of the stumps, especially those of the fir tribe, take fifty years 

 to rot away, though exposed in the air to alternations of rain and 

 sunshine, a fact on which every geologist will do well to reflect, for it 

 is clear that the trees of a forest submerged beneath the water, or still 

 more, if entirely excluded from the air, by becoming imbedded in sediment, 

 may endure for centuries without decay, so that there may have been 

 ample time for the slow petrifaction of erect fossil trees in the Car- 

 boniferous and other formations, or for the slow accumulation around 

 them of a great succession of strata." 



In another place, in speaking of the trees in the Great 

 Dismal Swamp, Lyell writes: — "When thrown down, they are 

 soon covered by water, and keeping wet they never decompose, 

 except the sap wood, which is less than an inch thick 2." We 



1 Lyell (45) vol. i. p. 60. 2 Lygii (45) vol. i. p. 147. 



