686 W. J. MILLER PRE-CAMBRIAN FOLDING IN NORTH AMERICA 



LACK OF RECORDS FOR MUCH OF THE CONTINENT 



Vast areas of North America contain few, if any, outcrops of definitely 

 known pre-Cambrian rocks, as, for example, Alaska, Mexico, the Cor- 

 dilleran region and plains of western Canada, the western Cordilleran 

 region of the United States, the Great Plains, much of the Mississippi 

 Valley, and the Atlantic and Gulf coastal plains. Even within the vast 

 pre-Cambrian area of Canada observations on the rock structures are 

 widely scattered, and there are hardly any for its northern half. 



Still another difQculty lies in the fact that in many areas containing 

 pre-Cambrian rocks descriptions of the rocks are very brief, and one 

 often looks in vain for such structural data as the strike of folds and 

 foliation. 



This lack of evidence in regard to so much of the continent is a very 

 serious difficulty in the way of trying to determine any possible uni- 

 formity or systematic arrangement of trend-lines or strikes of folding 

 and foliation of pre-Cambrian age, the more so because, where such 

 strikes have been rather definitely determined in many districts, they 

 seem to show anything but a systematic or uniform grouping (see accom- 

 panying map). 



SIGNIFICANCE OF THE DURATION AND SUBDIVISIONS OF THE PRE-CAMBRIAN 



It is quite generally recognized that known pre-Cambrian time was 

 tremendously long, probably fully as long as all later time put together. 

 It is also generally agreed that there were various periods of more or 

 less vigorous diastrophism, often accompanied by folding, in pre-Cam- 

 brian time, and that diastrophic forces were, as a rule, less pronomiced 

 in the later than in the earlier pre-Cambrian. Also, in a very general 

 way, the pre-Cambrian has commonly been divided into two great eras — 

 Archeozoic and Proterozoic. It must, however, be recognized that these 

 two subdivisions by no means represent such definite time intervals as 

 do the Paleozoic, Mesozoic, and Cenozoic eras. Where, for example, both 

 the Archeozoic and Proterozoic rocks occur in a given region, the line 

 of separation may represent a time many millions of years younger or 

 older than the line separating rocks similarly classified in some other 

 region. A perusal of the literature shows that this is a real difficulty in 

 the classification of the pre-Cambrian rocks, even of the Canadian Shield 

 itself, in regard to which various writers are by no means in agreement 

 as to where the line (unconformity) between the older and the younger 

 pre-Cambrian rocks should be placed, and some writers even argue for 

 more than two important groups of rocks. 



