CHAPTER Ty! 
FOSSILS 
Modes of Preservation of Organic Remains. Kinds of Rock in which 
Fossils occur. Fossils chiefly of Marine Origin. Importance of 
Fossils in Geological Investigations. Climatic and Geographical 
Conditions and Terrestrial Movements deduced from Fossils. 
Geological Chronology and Fossils. 
HITHERTO we have been concerned with rocks mainly as 
aggregates of mineral matter, and only a passing reference 
has been made to the fact that certain derivative accumula- 
tions contain /fossz/s—the remains and traees of formerly 
living creatures. We have seen, it is true, that some kinds 
of rock, such as coal and limestone, consist chiefly of the 
débris of plants and animals, but we have now to realise that 
almost every variety of derivative rock may be more or less 
fossiliferous, and that traces of former life have been met 
with, now and again, even in certain igneous and metamorphic 
rocks. 
When a plant or animal, or any portion of either, is buried 
in sediment, it becomes subject to decomposition. This 
process usually results in the destruction of all organic com- 
pounds of carbon and nitrogen, and even the harder and 
more durable parts undergo some change, and may eventually 
become disintegrated, and entirely disappear. Certain 
chemical changes, however, may supervene before the process 
of destruction is completed. In many cases, for example, 
carbonisation takes place—various gases are given off, and 
the organic tissues are-gradually transformed into carbon. Or 
mineral matter may be introduced in solution so as to fill up 
all the cavities of the original structures, or even to replace 
completely the substance of the organism, Fossils, therefore, 
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