in the siliceous petrifactions (Barghoorn, 1960), particularly in 
the opalized forms (Buurman, 1972). Moreover, silica, when 
crystalline, is among the least weatherable of all the minerals of 
common occurrence. In addition to being resistant chemically 
to most attacking solutions, its hardness and lack of cleavage 
help it to resist many other agencies of weathering (Deer et al., 
1966). Carbonates are considerably less durable, being com- 
parably soft and subject to dissolution, especially when in 
contact with CO:z-bearing ground waters. Sulfides, in the pres- 
ence of atmospheric oxygen and moisture, form sulfuric acid 
which causes rapid deterioration of the petrifaction and even- 
tual obliteration of all histological detail. 
Occasionally, woody relics of a former forest ecosystem are 
preserved en masse, usually as an assemblage of silicified 
petrifactions. At Amethyst Mountain in Yellowstone National 
Park, there is a time succession of at least twenty-seven such 
forests (Dorf, 1964). At this locality, many of the tree trunks 
stand upright, in their original position of growth. More often, 
as is the case with the Chinle forest of Arizona, fossil forests 
represent accumulations of drift wood which became silicified 
at the ‘‘forest’’ site after transport by ancient streams from 
elsewhere. Petrified woods are unrestricted geographically. 
They are found in many parts of the world and in a variety of 
physiographic provinces, from the tropics to the polar regions. 
They occur in sediments ranging in age from the Devonian, 
some 400 million years ago, to as recent as the latter half of the 
past century. 
Commonly, the internal organic architecture of the original 
wood substance is retained in the petrifaction in sufficient 
detail to permit thorough anatomical description. Woods of 
many diverse taxonomic groups are known in petrified form. In 
the Eocene fossil forests of Yellowstone Park alone, more than 
ten genera of coniferous woods and over thirty genera of dico- 
tyledonous woods have already been recognized (E. Wheeler, 
personal communication). Considerable diversity among pet- 
rified woods is found in the extent to which and manner in 
which mineral substance is emplaced in vascular tissue. Va- 
riety exists between those in which mineral matter is present 
within the confines of the cell walls and continuous across 
tissue, to others in which only the larger void spaces (cell 
lumina and fracture openings) are plugged with mineral sub- 
stance (see section III). 
7 
