121 TEXTBOOK OF BOTANY 



similar to those formed when leaves fall upon freshlv laid cement and 

 a detailed outline of their form and venation is preserved as the cement 

 solidifies. 



Fig. 358. Positive and negative imprints of a seed fern {Neuropteri.s) leaflet. 

 Photo from Field Museum of Natural History. 



Fossils may be found in consolidated rocks, sand dunes, cave de- 

 posits, peat beds, coal, river deltas and varves, ooze at the bottom of 

 lakes and oceans, glacial drift, and in soil formed from the weathering 

 of rocks. Studies of the rocks and fossils within them have enabled 

 paleontologists to reconstruct many phenomena and organisms of the 

 different geological periods (Fig. 359). 



Various means are used to estimate the relative and approximate 

 ages of fossils. For long-range estimates, the determination of the ex- 

 tent of disintegration of uranium to lead and helium appears to be far 

 more reliable than any of the methods formerly de^'ised. The rate of 

 this chemical process is not affected by the ordinary changes in tem- 

 perature and pressure at the surface of the earth. Since the rate of 

 disintegration of uranium is known, it is necessary only to determine 

 the amount of "uranium-lead" in a rock to estimate its age. 



The plants of a billion years ago were less complex than those of 

 today, and apparently were neither very large nor woody. They too fell, 

 and some of them were deposited in situations where oxidation and dis- 

 integration were delayed for at least sufficient time for casts and imprints 



