THE PROTEROZOIC ERA. 163 



important to emphasize the inauguration of the reign of these processes, 

 for they have been the dominant ones ever since. It is not to be over- 

 looked, however, that the vuJcanism which had predominated in the 

 Archean era was still a factor, though a declining- one. Locally it still 

 made larger contributions than sedimentation to the formations of 

 the era, and such has been its habit in all later times. But; taken as a 

 whole, Proterozoic times, as the descriptions of the formations will show, 

 were marked by more igneous activity than the Paleozoic, Mesozoic, 

 or Cenozoic eras that follow, and the Proterozoic may therefore be 

 regarded as a transition- time between the profoundly igneous era that 

 preceded, and the markedly sedimentary eras that followed. 



Stratigraphic Relations of the Proterozoic Rocks. 



The Proterozoic series are generally separated from the Archean 

 below and from the Paleozoic above by unconformities (Vol. I, p. 18). 

 Unconformities vary greatly in extent and significance. Great uncon- 

 formities usually involve three elements: First, a change of attitude 

 in the lower formation, as the result of which it is subject to erosion; 

 second, a period of erosion during which its upper part is truncated; 

 and third, another change resulting in the deposition of the upper 

 series on the eroded surface (Figs. 49 and 50). In some cases the 



Fig. 49. — Diagram showing Archean land (At) with sedimentation, a, along its bor- 

 ders. The coarser sediments are being deposited near the shore; the finer, farther 

 from it. (Compare Fig. 50.) 



unconformity may be due, theoretically, to a withdrawal and return 

 of the sea, or to a lifting and subsidence of the land, or to both acting 

 in cooperation. But if the beds below the unconformity are deformed, 

 it is obvious that more than mere change of level was involved. In 

 most cases the unconformity between the Archean and the Proterozoic 

 systems indicates that the former was notably deformed, and after- 

 wards subjected to prolonged erosion, before the deposition of the latter. 

 It is usually an invasion of the sea which furnishes the conditions for 



