72 THE OLD SHIELDS 



hemisphere we find corresponding shields in Brazil, South Africa and 

 Australia. There are smaller detached areas, such as Madagascar 

 and India. Anyone familiar with geology knows that it is just those 

 areas which have formed the subject of much theorizing about their 

 former position. 



STABILITY OF OLD SHIELDS 



This again we may leave to the specialists, and come back to our 

 statement that only within these old shields can vestiges of the origin 

 of life on earth, and of the early life itself, be found. Everywhere 

 else younger rocks have covered the pre-Cambrian basement. In 

 these younger areas we now may distinguish regions where the crust 

 remained relatively stable, and the pre-Cambrian basement was cover- 

 ed only by a not too thick pile of sediments. These regions, which 

 more or less surround the old shields, are left blank in Fig. 15. 

 Elsewhere the crust has subsided much more strongly. This resulted 

 in the formation of definite geosynclinal basins, which became filled 

 with very thick piles of sediments, and subsequently developed into 

 the fold belts of orogenetic cycles. Three of the most important of 

 these orogenetic cycles since the beginning of the Cambrian have 

 been indicated in Fig. 15. 



From this presentation it follows that the old shields, and also 

 large areas around them, remained more or less stable during later 

 history. New orogenetic cycles often develop outside the former ones, 

 leaving the older terrain undisturbed. The older areas are, in some 

 mysterious way, solidified. A rather naive comparison is often made 

 between the sediments folded during an earlier orogenetic cycle, and 

 corrugated iron which also is stronger than sheet iron. Although this 

 comparison in all probability is unwarranted, it may well help to 

 visualize this distinction between areas of the crust apparently solid- 

 ified by earlier orogenies, and those which later became mobilized 

 and developed into geosynclinal basins in younger orogenetic cycles. 



This distinction is important in our search for areas where vestiges 

 of early life may be found. On the stable platforms of older oro- 

 genetic cycles, newer sediments may be deposited from time to time 

 in later history. But, these newer sediments remain thin when com- 

 pared to the thick piles of sediment deposited in geosynclinal basins. 

 Such a thin cover of younger sediments does not lead to such high 

 pressures, nor to a very much higher temperature in the underlying 



