512 GENERAL CONSIDERATIONS 



581. Geological Ages.— Since the same conditions have been many 

 times repeated in the history of the earth and the same minerals have 

 been present at all times, it is impossible to determine the age of a stratum 

 by its structure or composition. However, the study of fossils in strata 

 which remain in the same order as that in which they w^ere deposited 

 shows that the forms of life have changed from time to time. Conse- 

 quently, by critical study of the fossils and comparison of strata in dif- 

 ferent parts of the earth, geologists are enabled to recognize those strata 

 belonging to the different ages and to relate them in a sequence which 

 corresponds to the order of their deposition. From the thickness of the 

 strata, in connection with their character, an estimate can be made as to 

 the duration of past ages. This estimate may be confirmed or in some 

 cases modified by the application of knowledge of chemical changes which 

 involve a time factor capable of precise calculation. The last data have 

 in general considerably increased the length of time formerly allotted to 

 the different ages by geologists. Estimates of the duration of these 

 ages still remain, however, very uncertain. 



582. Geological Time Scale. — On the basis of the estimates referred 

 to in the preceding section, geologists have prepared a time scale, which, 

 as represented in North America, is shown in abbreviated form in Fig. 312. 



583. Metamorphism. — The rocks of the Archean period are uni- 

 versally distributed over the earth, though to a great extent they are 

 covered by more recent formations. (This is not true of the distribution 

 of those of any other period.) They show evidences of sedimentary origin 

 but are everywhere modified by metamorphism. Metamorphism occurs 

 when stratified sedimentary rocks become buried deeply by overlying 

 strata and are subjected to the internal heat of the earth. The rocks of 

 the Huronian and Keweenawan periods are more decidedly sedimentary 

 in character, but they still show much metamorphism. Those of the 

 Cambrian and Ordovician are less generally metamorphosed; and the 

 later periods show no such effect. The result of this metamorphism was 

 to destroy all fossils and with them most of the evidences of the existence 

 of life. In the Archean, beds of carbon in the form of graphite, of iron 

 ore derived from carbonates, and of limestone of sedimentary origin are 

 all indications of the presence of hving organisms in that period. The 

 few ill-preserved fossils found are all considered to be remains of algae. 

 In the Proterozoic are found fossil calcareous algae, bacteria, radiolarians, 

 sponges, fragments of crustaceans, and the tracks of marine annelids, but 

 these fossils are exceedingly rare and scattered. Owing to the general 

 metamorphism of rocks older than the Cambrian, therefore, geological 

 records of earlier life have been almost entirely obliterated. Conse- 

 quently, it is not surprising that in the oldest strata which contain fossils 

 in any considerable numbers, most of the invertebrate phyla are repre- 

 sented. The result is, however, that there are no evidences from pale- 



