PALEONTOLOGY AND GENETICS 



statements as that the Mesozoic Era began somewhere between 190,000,- 

 000 and 240,000,000 years ago. But the difference between these figures 

 is more than 25 per cent of the smaller one. In addition to this difficulty, 

 the strata have been changed by such geological processes as folding and 

 erosion, so that often the record is fragmentary and confused. There are 

 a few instances in which stratified rocks appear to have been laid down 

 in definite annual layers, the varves, more or less comparable to the annual 

 growth rings of trees. In such cases, the extent of the periods of sedimen- 

 tation can be determined with great exactness, if the varves actually do 

 represent annual layers. But this is by no means certain. And even if it 

 were certain, the number of known examples is so small that it could 

 have little importance for the general problem of dating geological history. 



Although the study of sedimentary rocks has not led to a satisfactory 

 dating system, it has been possible to determine the sequence in which 

 the various strata of the earth's surface have been laid down, from very 

 ancient rocks right up to those of very recent origin. Particular strata are 

 identifiable not only by their position and their physical characteristics, 

 but also by the fossils which they contain. Thus it has been possible to 

 divide geological time into a series of eras, the sequence of which is un- 

 doubted. The first two eras, the Archeozoic and the Proterozoic, are not 

 of great interest for the present discussion, because the rocks deposited in 

 these eras contain very few fossils, and those are generally of doubtful 

 character. During the Paleozoic Era, fossils were deposited in great abun- 

 dance, but only archaic types were present. At first, only invertebrates 

 were represented, but fishes, amphibians, and finally reptiles made their 

 appearance during the Paleozoic Era. The next great era was the Meso- 

 zoic, or Age of Reptiles, during which birds and small mammals also arose. 

 Finally, the Cenozoic Era, which is still in progress, has been marked by 

 the rise to dominance of the mammals and man. 



The eras are, however, immensely long expanses of time, characterized 

 l)y progressive differences of flora and fauna, and not infrequently of 

 climate and other physical characteristics. The eras are therefore divided 

 into periods of shorter, but still very long, duration. Thus the Paleozoic 

 Era lasted for about 300,000,000 years, but it is subdivided into seven 

 periods, the durations of which vary from 25,000,000 years to 80,000,000 

 years. Finally, it is sometimes desirable to break up the periods into still 

 smaller divisions, the epochs. The Tertiary Period of the Cenozoic Era, for 

 example, lasted for about 74,000,000 years. This is subdivided into five 

 epochs the durations of which varied from 11,000,000 years to 19,000,000 

 years. 



While this system of time measurement is not quantitatively accurate, 

 it is, on the whole, quite workable. For, if a fossil is found in strata from 

 the Cretaceous Period, it is possible to state with complete assurance that 

 it followed Jurassic forebearers and preceded Paleocene descendants, if 

 any descendants were left. The geological time scale, together with some 

 of the characteristics of life at each level, is summarized in Table 1. 



The Lead Method. In 1907, Boltwood introduced a method for dating 

 geological strata based upon radioactive elements. The conclusions to 



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