CONVERGENCE OF EVIDENCE 881 



Kaleviaii. At Ytterby the intrusions are given as Ser-archean and equiv- 

 alent in age to the post-Kale vian, but the Moss granites, with a well de- 

 termined age of 925,000,000 years, are also post-Kalevian. The data for 

 the Ytterby region are thus discordant, but they rest on two analyses 

 in which the lead is given by Holmes as only 0.18 and 0.36 per cent, re- 

 spectively, a lesser lead-nraninm ratio and consequent lesser age being 

 given by the higher and more reliable percentage. The Ytterby granites 

 may then be provisionally regarded as not properly belonging to the group 

 whose age is 1,125,000,000 years. 



The number of determinations is not yet large enough to make it clear 

 if the Precambrian granites over the world fall into a series of sharply 

 defined groups. If they do, the decay of uranium minerals will give the 

 most valuable method of Precambrian correlation. x4t present it appears 

 that the group of granites whose geological position is designated as post- 

 Kalevian, post-Huronian, or pre-Animikean is approximately 925,000,000 

 to 950,000,000 years old, and that the group variously designated as post- 

 Bottnian, post-Sudburyan, or post-Temiskaming has an antiquity of 

 1,125,000,000 to 1,150,000,000 years. 



Back of these lie still older granite-gneisses — the Laurentian system of 

 Canada, the post-Ladogian of Fenno-Scandia — and tliese in turn were 

 intruded into older sedimentary and effusive igneous series. It seems 

 probable, then, that the oldest known rocks are as much as 1,400,000,000 

 years of age. 



Beyond these most ancient milestones lies the Primordial era, whose 

 stratigraphic record has been destroyed by engulfment in magmas from 

 below and by repeated cycles of erosion from above. As to its length, 

 there is no indication other than that the oldest known rocks which mark 

 the beginning of the following era contain sediments which testify to an 

 earth surface on which air and water played their parts, much as in later 

 times. Crust, ocean, and atmosphere had by the opening of the Archeo- 

 zoic already attained a condition of stability. 



ADJUSTMENT WITH GEOLOGIC EVIDENCE 



With these various control points more or less definitely established by 

 means of the age of uranium minerals, a time table may be constructed 

 for the eras of fossiliferous rocks, using the evidences of the quantity of 

 erosion and of sedimentation to assist in establishing the relative lengths 

 of the periods. The periods of a single era may be thus measured, but it 

 has been argued that the mean rate of erosion and sedimentation for the 

 several eras was quite different. In the stratigraphic records of the older 

 eras there is, furthermore, a larger proportion of lost intervals. 



