GEOLOGIC TIME. 



333 



Archenn or pie-Algonkiaii time I have no estimate based on a study of 

 Archean strata to offer. If we assume Houghton's estimate of 33 per 

 cent for the Azoic period and 07 per cent for the sedimentaiy rocks, 

 Archoan time wouhl l)e represented by the period of 22,250,000 years. 

 In estimating for the Archean, Houghton inchided a large series of 

 strata that are now placed in the Algonkian of the Proterozoic of the 

 U. S. Geological Survey; and T think that his estimate is more than 

 one-half too large; if so, 10,000,000 years would be a fair estimate, or 

 rather conjecture, for Archean time. 



It is easy to vary these results by assuming different values for area 

 and rate of denudation, the rateof deposition of carbonate of lime, etc.; 

 but there remains after each attempt I have made that was based on 

 any reliable facts of thickness, extent, and character of strata, a result 

 that does not pass below 25,000,000 to 30,000,000 years as a mininnim 

 and 60,000,000 to 70,000,000 years as a maximum for post- Archean geo- 

 logic time. 



I have not referred to the rate of developmeni, of life, as that is 

 virtually controlled by conditions of environment. 



In conclnsion, geologic time is of great but not of indertnite <luration. 

 I believe that it can l)e measured by tens of millions, but not by single 

 millions or hundreds of millions, of years. 



DESCRIPTION OK MAP. 



On the map (Plate xvi) the hyimthetical areas of the Cordilleran, 

 Mississippian, and Appalachian seas are clearly indicated. The land 

 area west of the Cordilleran sea is numbered No.l, and the Californian 

 sea and the area of Paleozoic deposits of western British Colund)ia, 

 No. 10. The northern extension of the Cordilleran sea (No. 9) is con- 

 tinued as the Paleozoic-Devonian sea to the Arctic Ocean. The early 

 Cambrian land area (No. 2) east of the Cordilleran sea must have been 

 more or less covered by water during later Paleozoic time. The area 

 now covered by Mesozoic deposits, indicated by No. 3, was presumably 

 covered by the westward and northward extension of the Paleozoic- 

 Mississippian sea. The area east of the Appalachian sea is indicated 

 by No. 4; and the supposed land barrier between the Hudson Bay and 

 the Mississii)i)ian sea by No. (>; it is not improbable that daring Ordo- 

 viciau or Silurian time a sea may have conuecited the two latter seas. 



