NATURE OF THE GEOLOGIC RECORD 135 



mals have the least chance. In order to be preserved as a fossil the body of 

 a horse, for example, must be prevented from complete destruction. Not 

 only the softer portions of the body but also the skeleton will disintegrate in 

 a few years if exposed to action of predatory animals, scavengers, insect 

 larvae, bacteria, and the erosive forces of the weather. A dry climate fav- 

 ors preservation of bones, but even dry bones disintegrate in time. Hence it 

 is necessary that the bones be protected by being covered. Wind-blown soil, 

 such as that which produced the thick deposits of clay known as loess, may 

 provide the protective covering. Or if the animal becomes mired in a bog 

 or in quicksand the bones may gradually sink and be covered. The fossils 

 in the Rancho La Brea asphalt pits are a special case of this procedure. Or 

 if the bones happen to lie in the flood plain of a river they may be covered 

 by a deposit of soil left by the river when it overflows its banks in time of 

 flood. Or the river, in flood stage, may sweep the bones into a lake or into 

 the sea, where they will be mingled with the remains of aquatic animals. 

 Thus, one of the most productive sources of fossils of Tertiary mammals is 

 the White River Bad Lands of South Dakota and Nebraska. The material 

 of these beds was laid down as a delta by rivers flowing from the Black 

 Hills, mountains formerly much higher than they now are. Mammalian 

 remains were swept down by the rivers and became embedded in the 

 delta. 



We see, then, that the chances are against an animal's hard parts' being 

 fossilized, particularly if the animal is a land dweller. Of the animals which 

 are fossilized, how many will be known to geologists a million or more 

 years later? A first hazard facing these fossils consists of the chemical and 

 geologic processes at work on the deposits in which the fossils are em- 

 bedded. We have seen (p. 128) that the original material of the fossil is 

 more or less completely replaced by minerals transported by water per- 

 meating the deposit. The deposition of minerals may follow faithfully the 

 original structure, or the process of replacement may more or less com- 

 pletely obliterate that structure. If later deposits are piled on the one in 

 question fossils in the latter are almost certain to be crushed and distorted. 

 The consummation of such destruction is reached if the deposit finally 

 comes to lie deep in the earth's crust with thousands of feet of other de- 

 posits above it. As a result of the tremendous pressure and accompanying 

 heat the deposit may be so altered that all fossils in it are destroyed. Later 

 the various minerals will recrystallize in the crystalline forms characteristic 

 of each. Rock that has undergone this process of internal alteration through 

 heat and pressure followed by recrystallization is called metamorphic rock. 

 We have mentioned limestone as a prominent sedimentary rock rich in 



