IV PETRIFIED TREES. 79 



So far we have chiefly considered examples of plants pre- 

 served in various ways by incrustation, that is, by having been 

 enclosed in some medium which has received an impression of the 

 surface of the plant in contact with it. By far the most valuable 

 fossil specimens from a botanical point of view are however 

 those in which the internal structure has been preserved ; that 

 is in which the preserving medium has not served merely as an 

 encasing envelope or internal cast, but has penetrated into the 

 body of the plant fragment and rendered permanent the 

 organization of the tissues. In almost every Natural History 

 or Geological Museum one meets with specimens of petrified 

 trees or polished sections of fossil palm stems and other plants, 

 in which the internal structure has been preserved in siliceous 

 material, and admits of detailed investigation in thin sections 

 under the microscope. Silica, calcium carbonate, with usually 

 a certain amount of carbonate of iron and magnesium carbonate, 

 iron pyrites, amber, and more rarely calcium fluoride or other 

 substances have taken the place of the original cell-walls. Of 

 silicified stems, those from Antigua, Egypt, Central France, 

 Saxony, Brazil, Tasmania ^ and numerous other places afford good 

 examples. Darwin records numerous silicified stems in Northern 

 Chili, and the Uspallata Pass. In the central part of the Andes 

 range, 7000 feet high, he describes the occurrence of " Snow- 

 white projecting silicified columns... They must have grown," he 

 adds, " in volcanic soil, and were subsequently submerged below 

 sea-level, and covered with sedimentary beds and lava-flows ^" 

 A striking example of the occurrence of numerous petrified 

 plant stems has been described by Holmes from the Tertiary 

 forests of the Yellowstone Park. From the face of a cliff on 

 the north side of Ameythryst mountain " rows of upright trunks 

 stand out on the ledges like the columns of a ruined temple. 

 On the more gentle slopes farther down, but where it is still too 

 steep to support vegetation, save a few pines, the petrified 

 trunks fairly cover the surface, and were at first supposed by us 

 to be the shattered remains of a recent forest'." Marsh* and 



1 There is a splendid silicified tree stem from Tasmania of Tertiary age 

 several feet in height in the National Museum. 



'' Darwin (00) p. 317. =' Holmes (80) p. 126, fig. 1. * Marsh (71). 



