752 J. BARBELL MEASUREMENTS OF GEOLOGIC TIME 



The adjustment of these two lines of evidence serves as the basis for a 

 table of geologic time stated in years. Such a table is comparable to the 

 first crude measurements of the distances of the stars in space. The 

 progress of research will continually refine the determinations and lead 

 to a higher order of precision. On the present data Cenozoic time is de- 

 termined as between 55,000,000 and 65,000,000 years long; the Mesozoic 

 as covering between 135,000,000 and 180,000,000 years; Paleozoic time 

 endured between 360,000,000 and 540,000,000 years. The summation of 

 these minimum and maximum figures indicates that the beginning of the 

 Cambrian was between 550,000,000 and 700,000,000 years ago. This may 

 seem to be a wide latitude, but the ratio of the minimum to the maximum 

 is no greater than those estimates now current and based on stratigraphy. 

 The important conclusion is that time since tlie beginning of the Cam- 

 brian is from ten to fifteen times longer than has been generally accepted 

 by geologists. 



Surprising as it may seem, the date known with the greatest precision 

 lies far back in pre-Cambrian time. From N'orway, Texas, Quebec, and 

 German East Africa uranium minerals associated with granites give an 

 age which approximates to 1,120,000,000 years. In the Scandinavian 

 nomenclature the granite is post-Bottuian. In Quebec it is later than the 

 Lauren tian granites and is probably post-Sudburyian (post-Temiscam- 

 ing). 



This is the age, consec^uently, of the second great granitic invasion of 

 known geologic time. The Laurentian granites, the first great invasion, 

 may have an age as great as 1,400,000,000 years. 



These measurements of the length of the several eras give a basis for 

 estimates of the mean rates of erosion and sedimentation. In the pre- 

 Cambrian they point to the existence of long periods of quiet, during 

 which the continents were baseleveled. The profound revolutions, marked 

 by folding, magmatic invasion, and regional metamorphism, were rela- 

 tively brief periods closing long eras marked by diastrophic quiet and low 

 continental relief. 



Another aspect to be considered lies in the relations of organic evolu- 

 tion to this great expansion of time. If it be assumed that the lowest 

 forms of life began shortly after the earth's surface was fitted to support 

 them, then instead of evolution being compressed into less than a hundred 

 million years, it is stretched out over a period of the order of 1,500,000,- 

 000 years. A discussion is given of the biologic advantage of this far 

 greater duration for accomplishing the organization of the cell and the 

 estal)lishment ol' the principles of heredity. Although evolution may ad- 

 vance by saltatiojis and is accomplished through the establishment of new 



