INTRODUCTION 13 



It is thus seen that the present composition of a fossil is no 

 indication of its original composition, for there may be iron or 

 siliceous replacements (pseudomorphs) of calcareous shells, 

 iron pseudomorphs after chitinous skeletons and plants, and 

 even, as in some sponges, calcareous pseudomorphs after sili- 

 ceous skeletons. The determination of the original composi- 

 tion depends, in such cases, upon comparison of the form of the 

 organism with the nearest living representatives. 



Carbonization. — If leaves fall into water, they sooner or 

 later sink to the bottom, where they may be noted along the 

 tree-lined margin of any lake or quiet stream ; here compara- 

 tively little free oxygen can reach them. Decay accordingly 

 is very slow and usually but partial, leaving a residue of carbon ; 

 this, made fine by the motion of the water, gives the black color 

 so characteristic of such bottoms. The form of the leaves under 

 these conditions is, accordingly, not preserved, but their presence 

 is indicated by the black color still persisting after these ancient 

 lake and river margins have hardened into rock. Other leaves, 

 though suffering a similar concentration of carbon, fall in a sit- 

 uation that insures their more rapid burial by the mud brought 

 in by the rivers ; they are accordingly preserved as carbonized 

 films, showing all their original veining. This same process 

 would occur in the case of any other form of vegetation or even 

 of animals (Hydrozoa, Crustacea, fish, etc.), if they were buried 

 beyond the reach of those that prey upon them. 



The process of carbonization is essentially the giving off of 

 marsh gas (CH4), water (H2O) and carbon dioxid (CO2) ; the 

 last cannot be developed to the exhaustion of the carbon, since, 

 compared with air, little free oxygen exists in standing water. 

 Thus wood (6 CeHioOs) under such conditions would eventually 

 yield an anthracite coal (C24H8O). 



Good examples of carbonized plant fossils occur in the roof 

 shales of coal mines. Remains of animals similarly preserved 

 are likewise abundant ; well-known examples are Hydrozoa 

 (graptolites) in the Utica shales (Ordovician) of eastern New 



