Time-table for North America. 17 



probable diastrophic cycles of period value before geologists, 

 we have placed their names in the fourth column of the table. 

 In conclusion, we may truthfully say that there is now a 

 good deal of harmony among geologists in their use of the 

 theory that the surface of the earth is periodically and 

 rhythmically in motion, and that this diastrophic action is the 

 basis of chronogenesis, developing not only cycles of sea in- 

 vasion and land emergence, and cycles of erosion, but as well 

 cycles of organic evolution. Although the eras are clearly 

 recognizable everywhere, nevertheless, until the geologic geog- 

 raphy of Europe is worked out in detail, we shall not be able 

 to say that the various periods in current use are all established 

 in nature, and it will therefore be doubtless advisable for 

 America to continue to work out her own o-eologic chronologv. 



Part II. Pre- Cambrian Time (J. B.). 



"When the International Committee in 1905 proposed a 

 nomenclature for the pre-Cambrian rocks of the Lake Superior 

 region, it may have seemed to some at that time that the larger 

 relations had become fixed, and that future work, while not 

 altering the classification, would serve to develop details. The 

 classification then proposed represented in fact a large advance 

 upon that apparent hopelessness of solution of the pre-Cambrian 

 which in previous decades had been summed up in the name 

 of the ** Basement Complex,'' but the growth of knowledge 

 regarding this earliest division of earth historv has kept on 

 during the past decade with equal pace, and has been perhaps as 

 great as in any field of geology. It has revealed more clearly 

 a distant perspective of earth history analogous on a larger 

 scale to that vista of prehistoric human historv which has been 

 developed during this same decade in Europe. The meeting 

 of the International Geological Congress at Toronto in 1913 

 was the stimulus which determined that a revision of the pre- 

 Cambrian classification should be made, with the object of 

 making it express more exactly the present points of view. 

 This revision, however, like that of 1905,- must be regarded as 

 provisional only, another step toward a larger and more 

 accurate knowledge of the long eons which preceded the fos- 

 siliferous record. At least four somewhat different classifica- 

 tions were proposed, but, although showing some radical 

 differences, they nevertheless hold much in common. For the 

 purposes of this table, that of Coleman has been most largely 

 followed, but the views of Adams, Collins, Lawson, and M. E. 

 Wilson have also been of use. 



In arranging tables of geologic chronology, it has been 

 customary to show the sequence of stratified formations only. 



Am. Jour. Sci.— Fourth Series, Vol. XXXVIII. Xo. 223. -July, 1914. 

 o 



