FOSSILS: THEIR ORIGIN AND DISTRIBUTION. 



191 



Fig. 173.— Chalk-Cliffs. 



and deltas and buried in mud ; and tracks were formed on flat, muddy 

 shores by animals walking on them. These have been preserved with 

 more or less change, and are 

 even now found in great 

 numbers inclosed in strati- 

 fied rocks. They are called 

 fossils. A fossil, therefore, 

 is any evidence of the for- 

 mer existence of a living be- 

 ing. Fossils are the remains 

 of the faunas and floras of 

 previous geological epochs. 

 Their presence is the most 

 constant characteristic of 

 stratified rocks. 



Degrees of Preservation. 

 — Sometimes only the tracks 

 of animals, or impressions of leaves of plants, are preserved. More 

 commonly the bones or shells, or other hard parts of animals, are pre- 

 served with various degrees of change. Sometimes even the soft and 

 more perishable tissues are preserved. We will treat of these degrees 

 under three principal heads : 



1. Decomposition prevented and the Organic Matter more or less 

 completely preserved. — Cases of this kind are usually found in compar- 

 atively recent strata, and imbedded either in frozen soils, or in peat, or 

 in stiff clays ; although some cases of partial preservation of the or- 

 ganic matter are found even in old rocks. Extinct elephants have been 

 found frozen in the river-bluffs of Siberia so perfectly preserved that 

 dogs and wolves ate their flesh. Skeletons of men and animals are 

 found in peat-bogs and stiff clays of a comparatively recent formation, 

 the organic matter of which is still preserved. In clays of the Tertiary 

 period the imbedded shells still retain the epidermis, and even in the 

 Lias (Mesozoic) shells are found retaining the nacreous luster. Coal is 

 vegetable matter changed but not destroyed. It is found in almost 

 every formation, even down to the oldest. Every degree of change may 

 be traced in different specimens of fossil wood, between perfect wood 

 and perfect coal. 



2. Petrifaction: Organic Form and Structure preserved. — In the 

 last case the organic matter is more or less preserved. In the case 

 now to be described the organic matter is entirely gone ; but the or- 

 ganic form and the organic structure are preserved in mineral matter. 

 This is what is usually called petrifaction or mineralization. The best 

 example of this is petrified wood. In a good specimen of petrified 

 wood, not only the external form of the trunk, not only the general 



