Age of the Eart Ji . 3 6 7 



complete decomposition as in the Algonkian." (Pp. 21-22.) 

 Similar testimony is borne by Chamberlin and Salisbury 

 (' Text-book of Geology,' ii. p. 199). 



According to the definition of Algonkian and Archaean we 

 must draw a line at the base ot the former as representing 

 that limit at which geological time, as an era of sedimentation 

 and solvent denudation, began. "The Archaean was essentially 

 a period of world-wide vulcanism, and in the relative pro- 

 portions of rocks of igneous and sedimentary origin represents 

 a departure from the uniformity of conditions of later geolo- 

 gical times" (Van Hise and Leith, loc. cit. p. 30). 



Turning to the pre-Cambrian geology of other parts of the 

 world we find that the Torridonian and Lewisian of North- 

 west Scotland in their mutual relations and petrographical 

 characters resemble the Algonkian and Archaean divisions of 

 North America. The aggregate thickness of the Torridonian 

 lias been estimated at not less than 10,000 feet. To this the 

 Dalradian has, possibly, to be added. 



The pre-Cambrian rocks of Finland have been divided by 

 Sederholm as follows : — 



Jotnian. 



Jatulian, Upper. 

 Jatulian, Lower. 

 Kalevian, Upper. 

 Kalevian, Lower. 



Bottnian. 



Ladogian. 



Katarchsean. 



Sederholm makes the same statement regarding the Jotnian, 

 Jatulian, Kalevian, and Bottnian, as has been made with 

 reference to the Algonkian. 



Sederholm says: — "At least as far back as during Bottnian 

 time the climatic conditions were not sensibly different from 

 those of later geological periods, as shown by the existence of 

 rocks which, in spite of their metamcrphic character, show 

 themselves to be sediments with the same regular alternation 

 of clayey and sandy material (annual stratification) as the 

 glacial clays of that same region, explainable only by assuming 

 a regular change of seasons " (Sederholm, J. J.. Bull. Coium. 

 Gtvl de Finldnde, No. 23. p. 95, 1907). 



