MAJOR DIVISIONS OF THE PRE-CAMBRIAN 13 



relatiou.sliips, falling Into the same scheme of classification ; thus confirming 

 the idea that the rocks as grouped represent great and widespread systems, 

 separated hy profound alterations of geological conditions during great time 

 intervals." 



The Major Divisions of the pre-Cambrian 



In regions of America other than those ah-eady mentioned there was 

 Uttle attempt in early years to apply stratigraphic methods to the pre- 

 Cambrian rocks. They were regarded as an indivisible group. As already 

 noted, the men who thus treated the pre-Cambrian rocks as a unit first 

 used the term primary and primitive. Later the term Azoic gained favor, 

 on the ground that in these rocks no fossil evidence of distinct life had 

 been found, and it was believed that they were formed before life existed 

 upon the globe. This term contrasted the ancient rocks with the fossil- 

 iferous, and therefore zoic, formations. 



As knowledge of the pre-Cambrian grew, the protest against the as- 

 sum2)tion that life did not exist during this time became more and more 

 vigorous. To avoid the difficulty and to meet the needs of those who 

 M'islied to view the pre-Cambrian rocks as a unit, the name Archean, or 

 ancient, was suggested by Dana in 1872. This term very happily met 

 tlie needs of the general geologist, who did not wish to undertake detailed 

 work in the pre-Cambrian, and thus it raj^idly found favor. For many 

 years this term was Avidely used to include all the pre-Cambrian forma- 

 tions, and to the present time it is thus used by certain geologists. 



After the work of the Wisconsin and Michigan surveys was well ad- 

 vanced and after studies had been made in other parts of the world, prov- 

 ing that there are series of sedimentary rocks antedating the Cambrian, 

 which are radically different from a basement set on which they rest, 

 and when it was, moreover, shown that this set of sedimentary rocks was 

 one of large magnitude, Irving held that tlie great period of time repre- 

 sented by the upper pre-Cambrian group was long enough, so that it 

 should be recognized as of the first order — that is, it Avas wortliy of a 

 place of tbe order of Paleozoic and Mesozoic. He furtlior pointed out 

 tliat the roclvs of tliis time, while furnishing no definitely recognizable 

 fossils, contained life. This was conclusively shown by the large amount 

 of carbonaceous material and graphite in tlie slates, and it was believed 

 that the limestones and iron ores pointed in the same direction. Since 

 the nature of the life of this time was unknown, the name "Agnotozoic," 

 first suggested hj Chamberlin, was Ijrought forward by Irving, as was the 

 term "Eparchean." "Proterozoic" was also considered hy Irving for the 

 place, but was rejected by him. Agnotozoic gained a small place in liter- 

 ature, but Eparchean was never much used. However, to Agnotozoic it 



