GENERAL CONSIDERATIONS 687 



In many districts in Xorth America the oldest rocks are simply classed 

 as "pre-Cambrian"' not only because there is no valid reason for sub- 

 dividing them, but also because they can not, with any degree of assur- 

 ance, be classified as either Archeozoic or Proterozoic. Again, vrithin 

 groups of rocks classed as Proterozoic, the number of important struc- 

 tural breaks varies from none to several, and we have not a5 yet devised 

 a satisfactory method of correlating the subdivisions in one region with 

 those in a separate area far from it. These are all fundamental facts 

 which must be duly considered in any effort to work out the pre-Cam- 

 brian structural history of the continent. 



Evidence has already been presented to show that not all pre-Cambrian 

 rocks were highly folded in pre-Cambrian time. All through his paper, 

 however, Euedemann considers "pre-Cambrian worldwide folding""^ to be 

 an established fact. He also recognizes the great length of pre-Cambrian 

 time, and believes that the assumed worldwide pre-Cambrian folding 

 (pages 129-134) originated either from a "long-time, gradual extension 

 of local folding^' or from a worldwide simultaneous folding, more likely 

 the former. In any case he advocates the hypothesis that the pre-Cam- 

 brian continent of Xorth America responded to universal folding as a 

 unit (page 75), and that the diastrophic agencies developed a uniformity 

 of trend of the folded structures as indicated on the accompanying map. 

 The writer dissents from all three factors of this hypothesis for the fol- 

 lowing reasons : 



First. Evidence has already been presented to show that even very old 

 pre-Cambrian rocks have not been universally notably folded. 



Second. With regard to the proposition that the continent reacted as 

 a unit against folding agencies, it seems to the writer that such a con- 

 clusion is not warranted by the facts. In the light of our great uncer- 

 tainties concerning the subdivisions and correlation of subdivisions of 

 pre-Cambrian time, is it not dangerous to lump together the sum of 

 many local foldings of various times (geologic ages apart) as if they 

 represent a continuous, general process? If they do not represent such 

 a continuous, general process, then the sum total of all pre-Cambrian 

 folded structures can not possibly be used to represent the configuration 

 of the continent as a unit. In this connection we should think of the 

 various great diastrophic movements, widely separated in space and time, 

 which are definitely known to have taken place in Xorth America in 

 post-Cambrian time. During this vast lapse of time, probably no longer 

 than that of the pre-Cambrian, the continent has undergone many im- 

 portant diastrophic changes. As emphasized by Schuchert in his presi- 



