THE EVIOENCE FOR EVOLUTION 'J {J 



Figure 35.1. An example of a fossil, the remains of Archaeopteryx, a tailed, 

 toothed bird from the Jurassic Period. (Courtesy of the American Museum of Natural 

 History.) 



Molds and casts are superficially similar to petrified fossils but are 

 produced in a different way. Molds are formed by the hardening of the 

 material surrounding a buried organism, followed by the decay and re- 

 moval of the body of the organism. The mold may subsequently be 

 filled by minerals which harden to form casts which are exact replicas 

 of the original structures. Some animal remains have been exceptionally 

 well preserved by being embedded in tar, amber, ice or volcanic ash. 

 The remains of woolly mammoths, deep frozen in Siberian ice for more 

 than 25,000 years, were so well preserved that the meat was edible! 



308. The Geologic Time Table 



Studies of the earth's crust have shown that it consists of sheets of 

 rock lying one on top of the next. There are five major rock strata and 

 each of these is subdivided into minor strata. These layers were gen- 

 erally formed by the accumulation of sediment— sand or mud— at the 

 bottom of oceans, seas or lakes. Each rock stratum contains certain 

 characteristic kinds of fossils which can now be used to identify deposits 

 made at the same time in different parts of the world. Geologic time 

 has been divided, according to the succession of these rock strata, into 

 eras, periods and epochs (Table 15). The duration of each period or 

 epoch can be estimated from the thickness of the sedimentary deposits, 

 although, of course, the rate of deposition was not exactly the same in 

 different places and at different times. 



