164 GEOLOGY. 



deposition, particularly where the deposits form a wide-spread and 

 thick series; but this is not necessarily the case, for lodgment may 

 take place in interior basins and on low gradation-planes. In such 

 systems as the Proterozoic, where fossils are generally wanting, it 

 cannot be asserted that the sea was involved, though the probabilities 

 favor a marine origin for most of the sediments. 



Though the Proterozoic rocks are generally unconformable on the 

 Archean, unconformity between the two systems cannot be asserted 

 to exist at every point where their contact has been seen. It is, how- 

 ever, so general as to show that wide-spread changes of attitude (prob- 

 ably warpingslof jth^i crust) took place after the formation of the 

 Archean, and before the formation of the known Proterozoic. Great 

 lapses of time were 'doubtless involved in these changes, but of the 

 history of the interval recorded by the unconformity there is little 



V.. ' ''■ . *Sea Lei/e/ 



Fig. 50. — Diagram representing the same region as Fig. 49, after subsidence. The a of 

 this figure corresponds to a of Fig. 49. It is to be noted that fine sediments 

 overlie the coarse sediments deposited at an earlier time in the right-hand part 

 of the figure. 



more than conjectural knowledge. It is clear, however, that where 

 unconformity exists, the Proterozoic beds above the unconformity 

 are not the oldest sedimentary beds derived from the Archean, for 

 the material removed from the eroded Archean surface was deposited 

 somewhere, and that before the beds immediately above the plane 

 of unconformity were laid down. 



While unconformity between the Archean and the Proterozoic 

 is the rule 1 ; so far as their contacts are known, it should be noted that 

 such unconformity is presumably not universal. There were probably 

 places, even on the land- areas, where the surface of the Archean did 

 not suffer notable erosion before the deposition of Proterozoic sediments 

 upon it, and there were quite certainly such places in the areas con- 

 tinuously covered by the sea. Relations of conformity between the 

 two systems may therefore exist over great areas where their contact 

 is not exposed, and perhaps in some places where it is. 



