PRE-CAMBRIAN FORMATIONS— ONTARIO AND MANITOBA 469 



On the other hand, the original classification suggested by 

 Lawson, in which the sedimentary Coutchiching is the oldest 

 formation, cannot be applied to those successions in which the 

 sediments are interbanded with lava flows or even lie above rocks 

 which are lithologically similar to the Keewatin. 



From theoretical considerations it seems unlikely that any of 

 these formations can be used to correlate successions in different 

 districts. Commonly the basic flows have been used in this way. 

 Since they are lavas, it is impossible that any one eruption could 

 have extended to any great distance, and hence correlation on the 

 basis of lithology of separate flows must be most uncertain. Nor 

 is this affected by the possibihty that many of the flows are sub- 

 aqueous, as there is no evidence that the bodies of water beneath 

 which the flows may have been extruded were large or continuous. 

 In fact, in some instances the interbedding of ellipsoidal flows and 

 shallow water or terrestrial sediments is evidence that the bodies 

 of water were limited in area and of brief duration. Correlation 

 by means of the sedimentary beds is even less reliable. The con- 

 glomerate, slate, and gneiss of this early period are beheved to be 

 almost entirely of terrestrial or shallow-water origin. No bed 

 formed in this way could be expected to have great lateral extent, 

 and no determination of age can be made on the ground of its 

 similarity to rocks in other districts. 



Since no erosion break has been recognized in any of the suc- 

 cessions yet worked out, and since there is this very marked differ- 

 ence in the relations of sedimentary and igneous rocks in various 

 areas, it is plain that no course is possible, at present, except the 

 interpretation of the early part of the pre-Cambrian as a period 

 of volcanic activity in which eruptions of lava alternated with 

 deposition of ordinary clastic sediments. These periods of erup- 

 tion were recurrent, but not necessarily contemporaneous even in 

 neighboring districts. Hence the succession of volcanic and 

 sedimentary rocks is naturally not the same in any two districts. 

 The result is a great thickness of lavas, tuffs, and sediments, all of 

 which belong to one great period in the earth's history. It is 

 manifestly impossible to apply to rocks of such origin either of the 

 terms Keewatin or Coutchiching and, if the view set forth here be 



