OLD SHIELDS AND YOUNGER CRUSTAL AREAS 71 



rocks outcropping at the surface of the earth, without a cover of 

 younger rocks. 



In most textbooks of geology the old shields are indicated as quite 

 distinct from the newer regions of the earth's crust. If we look, for 

 instance, at Plate V of Umbgrove's Pulse of the Earth, which is 

 redrawn here in Fig. 15, we find the old shields indicated by stip- 

 pling, without any indication of pronounced structural directions. 

 This is in marked contrast to the newer regions of the crust, where 

 the main directions of younger orogenetic fold belts are indicated. 



This difference, suggested in most geology textbooks, between the 

 old shields and the younger areas of the crust, is but an apparent 

 one. It is brought about by the apparent break in classical geology 

 between all of the pre-Cambrian and the younger history of the 

 earth. Now, as we have seen already in Chapter III, the only dif- 

 ference is that from that time onwards we find enough fossils to 

 permit a relative dating of the younger rocks. In contrast, the pre- 

 Cambrian has only been devided, very loosely, into Archaeic and 

 Algonkian, or into Azoic and Proteozoic. These divisions are, how- 

 ever, practically meaningless for lack of fossils. Correlations of pre- 

 Cambrian rocks found on different continents are without any factual 

 foundation. 



Only now, with the use of absolute dating techniques, has the un- 

 ravelling of the structure and the history of the old shields become 

 a possibility. The reader should be very wary, consequently, of any 

 older conclusions about age relationships of rocks from any of the 

 old shields. Moreover, as may be said of other fields too, such con- 

 clusions are often the more sweeping, the fewer the facts they are 

 based on. 



The apparent difference between the old shields and the younger 

 areas of the crust is not that the old shields are so different in 

 structure, but that we know next to nothing about them. That the 

 old shields are indicated simply by stippling does not mean an 

 absence of structures in these shields, but that we do not know how 

 to correlate local structures into an overall picture. 



So let us just point out here the location of the old shields, as 

 given in Fig. 15. In a certain sense, the old shields form the cores 

 of the continents. There are a Canadian, a Fennoscandian and a com- 

 plex Asian shields on the northern hemisphere. On the southern 



