THE ARCHEOZOIC ERA 447 



The question of the origin of the Archean must, therefore, still be 

 regarded as an open one. 



Earlier views concerning the Archean. In explanation of the 

 Archean system, many different hypotheses have been suggested 

 at one time and another, most of them starting with the Laplacian 

 hypothesis as a beginning. One of them is that the Archean rocks 

 are wholly of metamorphosed sediments; a second, that they are 

 igneous rocks produced by the fusion of sediments; and a third, 

 that they are igneous rocks intruded beneath the oldest known 

 sedimentary rocks after the deposition of the latter. These hypoth- 

 eses have a historic interest, but since they are not now generally 

 held by geologists, their consideration will be omitted. 1 



Life during the Archeozoic era. The presence of carbonaceous 

 material, of bedded iron ores that were once carbonates, of cherts, 

 and of limestones, implies the presence of life during the era occu- 

 pied in the formation of the Archean rocks. Since no fossils have 

 been found, nothing is positively known of the character of the 

 life, and little, except by inference, of its abundance. 



Duration of the Archeozoic era. Of the duration of the Arche- 

 ozoic era nothing can be said beyond the general statement that it 

 was very great, a conclusion which is independent of any particular 

 conception of the earth's origin. If the planetesimal hypothesis is 

 the true one, there is no readily assignable lower limit to the Arche- 

 ozoic system, and the duration of the Archeozoic era may exceed 

 that of all subsequent time. 



Climate. Of the climate of the era nothing is known except 

 that it seems to have been such as to permit the existence of life, 

 and the ordinary phases of sedimentation. 



1 See the authors' Earth History, Vol. II. 



