856 



J. BARBELL MEASUREMENTS OF GEOLOGIC TIME 



IV. Turning to the Precambrian rocks of Scandinavia, there are three 

 series of igneous intrusions containing radioactive minerals. All are in- 

 trusive into the older schists and quartzites of the Precambrian, and all 

 were worn down by the denudation which prepared the platform on which 

 the Jatulian formations were laid down. They all, therefore, belong to 

 the middle division of Precambrian time. The position of these rocks 

 in the Precambrian sequence will be made clear by the following tentative 

 classification : 



Fenno-Scandia 



f Rapakivi granite 

 J Jotnian 



L Jatulian 



Upper 

 Precambrian 



or 

 Ep-Archean 



Ep-Archean interval 



C Ser-archean and 

 Kalevian granites 

 Upper Kalevian 



4 



Middle 

 Precambrian 

 or 



Mes-Archean 



Lower Kalevian 



Post Bottnian granites 

 Bottnian 



Epi-Laurentian interval 



Lower f Post-Ladogian granite 



Precambrian 



or 

 Prot-Archean 



1 Ladogian 



Canada 



Keweenawan 

 Animikie 



Algoman and post 

 Huronian granites 

 Upper Huronian 



Lower Huronian 



Granite intrusions 

 Sudburyan 



Laurentian granites 

 Grenville series 

 Keewatin series 

 Coutchicliing series 



The first group of minerals — from the Moss district of south Norway — 

 belongs to pegmatites associated with granites of post-Kalevian age. The 

 lead ratios only vary from 0.12 to 0.15 and the atomic weight of lead 

 from Broggerite (lead ratio = 0.13) was found by Honigschmid to be 

 206.06. Here, then, all the criteria of a thoroughly satisfactory series of 

 minerals are fulfilled, and the age assigned to the rocks may be accepted 

 with confidence. 



V. A similar suite of minerals is found in the pegmatites of the Arendal 

 district of south Norway. In this case the associated granites may be 

 post-Kalevian or post-Bottnian. The agreement of the lead ratios is good 

 except for the abnormal value given by Xenotime. (It is to be noted, how- 

 ever, that the per cent of lead in this mineral is only 0.62, which indicates 

 that but relatively little weight should be attached to the analysis. — J. B.) 



VI. Although many analyses have been made of minerals from the 

 famous pegmatites of Ytterby (Ser-archean granites) no determinations 

 of lead have hitherto been published. Two analyses by the writer indi- 

 cate that the miiu^rals are of the same order of age as those of the two 



