720 GENETICS AND EVOLUTION 



Tlie layers of sedimentary rock should occur in the sequence of 

 tiieir deposition, with the newer strata on top of the older ones, but 

 subsequent geologic events may have changed the relationship of the 

 layers. Not all of the expected strata may occur in some particular re- 

 gion, for that land may have been exposed rather than submerged 

 timing one or more geologic ages. In some regions the strata formed 

 previously have subsequently emerged, been washed away, and then 

 relatively recent strata have been deposited directly on very ancient 

 ones. Certain sections of the earth's crust, in addition, have undergone 

 massive foldings and splittings, so that early layers come to lie on top of 

 later ones. 



Rock deposits are now dated largely by taking advantage of the fact 

 that certain radioactive elements are transformed into other elements 

 at rates which are slow and essentially unaffected by the pressures and 

 temperatures to which the rock has been subjected. Half of a given 

 sample of uranium will be converted to a special isotope of lead in 4.5 

 billion years. Hence, by measuring the proportion of uranium and lead 

 in a bit of crystalline rock, its age can be measured. In this way the 

 oldest rocks of the earliest geologic period are calculated to be about 

 3,500,000,000 years old and the latest Cambrian rocks to be 500,000,000 

 years old. Events in more recent times can be dated quite accurately by 

 the decay of carbon^^, which has a half life of 5568 years.* 



Between the major eras there were widespread geologic disturbances, 

 called revolutions, which raised or lowered vast regions of the earth's 

 surface and created or eliminated shallow inland seas. These revolutions 

 produced great changes in the distribution of sea and land organisms 

 and wiped out many of the previous forms of life. The Paleozoic era 

 ended with the revolution that raised the Appalachian mountains and, 

 it is believed, killed all but 3 per cent of the forms of life existing then. 

 The Rocky Mountain revolution (which raised the Andes, Alps and Hi- 

 malayas as well as the Rockies) annihilated most reptiles of the Mesozoic. 



309. The Geologic Eras 



Archeozoic Era. The rocks of the oldest geologic era are very 

 deeply buried in most parts of the world, but are exposed at the bottom 

 of the Grand Canyon and along the shores of Lake Superior. The oldest 

 geologic era, the Archeozoic, begins not with the origin of the earth 

 but with the formation of the earth's crust, when rocks and mountains 

 were in existence and the processes of erosion and sedimentation had 

 begun. The Archeozoic era lasted about two billion years, about as long 

 as all the succeeding eras combined. It was characterized by widespread 

 volcanic activity and large upheavals which resulted in the raising of 

 mountains. The heat, pressure and churning associated with the move- 

 ments of the earth's crust probably destroyed most of whatever fossil 

 remains there may have been, but a few traces of life remain. Scattered 

 through the Archeozoic rocks are flakes of graphite, pure carbon, which 



* Organic carbon is derived by CO2 fixation from atmospheric CO2 and the ratio of 

 C12 to Ci* in living organisms is the same as that in the atmosphere. No exchange of 

 carbon atoms with the atmosphere occurs after death and the Ci* in the body is slowly 

 transformed into Ni*. The age of organic remains can be estimated from their Ci^/Ci^ 

 ratio and the half life of Ci*. 



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