at one extreme of the spectrum of organic decay there 
are the various processes which operate in the soil and 
lead to the formation of soil organic matter, while at the 
other extreme there are the conditions of anaerobic sub- 
mergence. In all cases microbiological processes induce 
physical and chemical changes in the plant tissues in- 
corporated in the substratum, but the degree of change 
and the duration of the change vary profoundly. 
Some understanding of the degradation of plant tissue 
is essential to a satisfactory interpretation of many types 
of plant fossils, since the variously modified remains of 
plant parts constitute the bulk of our knowledge of the 
paleontological record of plant life. For the most part 
these source materials are fragments or occasionally en- 
tire plants which fortuitously entered basins of deposition 
in which degradative processes were retarded and event- 
ually inhibited. Under certain unusual and poorly known 
conditions the infiltration of mineral salts in solution and 
their subsequent precipitation resulted in an unusually 
perfect preservation of the original structure of the plant 
tissues. This is well known in the case of many fossil 
woods and other plant parts which are silicified or calci- 
fied. The sequence of events preceding mineralization, 
however, appears to have been basically similar to that 
which prevails in any accumulation of plant fragments, 
as in the formation of peat and other carbonaceous sedi- 
ments. There seems to be no evidence that special envi- 
ronmental conditions have been responsible for the ‘‘bio- 
logical fixation’’ of mineralized plant fragments, i.e., 
their unusual retention of organic structure. Rather, the 
unusual conditions have been those leading to precipita- 
tion of the mineral matrix itself, as in the calcareous 
nodules, or ‘‘coal balls,’’ of certain coal seams, and the 
formation of silicified nodules. 
In anticipation of the evidence to be shown later in 
[3 ] 
