yS NEW YORK STATE MUSEUM 



What these deformative forces may have been can be more advan- 

 tageously discussed after the trend of the folds and associated 

 phenomena has been traced over the earth. Before, however, we 

 can take up this most important part of our inquiry, the question 

 of the correlation of the Precambrian rocks of the different continents 

 has to be settled ; for it would clearly be an idle undertaking to attempt 

 to trace the general direction of the strikes of these rocks on the 

 earth, if the probability of entirely different ages of the rocks and 

 of their diastrophic movements on the different continental plat- 

 forms had to be inferred. 



American geologists seem to be in full agreement that at least 

 two eras have to be distinguished in the Precambrian, that are 

 separated by a great unconformity. These are the Archeozoic and 

 Proterozoic eras. The former term is applied to the dominantly 

 igneous or meta-igneous complex below and the latter to the domi- 

 nantly sedimentary or metasedimentary series above (see Chamberlin 

 and Salisbury, 1909, Pirsson and Schuchert, Miller and Knight 

 19 1 5, Schuchert, 19 16). The sedimentary rocks of the Archeozoic 

 era (Archean rocks) are known as the Grenville series; the igneous 

 series as the Keewatin. The Archeozoic era closed with the great 

 Laurentian revolution, which thoroughly deformed and injected 

 the Archean rocks with the Laurentian granite-gneiss. This was 

 followed by an erosion interval of immense duration, the Ep-Archeo- 

 zoic interval. In the rocks of the Proterozoic era there can again 

 be recognized a great unconformity. The Proterozoic era has there- 

 fore been subdivided into the Huronian and Algonkian periods. 

 Schuchert (1916, p. 479) proposes to recognize these two periods as 

 eras, in recognition of the fact that the Huronian again closes with 

 a great mountain-making revolution and a period of great granite 

 intrusions (the Algoman intrusives), which produce a major uncon- 

 formity such as he considers as diagnostic of the termination of an 

 era in the geologic succession. For reasons of priority he would 

 term the earlier one (also known as Timiscamian) , the Agnotozoic, 

 and the later (also termed Algonkian and Animikean, see Miller 

 and Knight, p. 592) the Proterozoic era. 



While the rocks of the lower Huronian or Agnotozoic era are still 

 highly metamorphosed, those of the later stages of the Proterozoic 

 era are metamorphosed scarcely at all. They likewise have under- 

 gone much less violent diastrophic movements than those of the 

 preceding eras and are sometimes fossiliferous. They have therefore 

 been even referred to the Paleozoic (Lawson, 19 16), although separated 

 from the Paleozoic system by another great unconformity and cor- 

 responding great erosion interval. 



