THE PROTEROZOIC ERA. 



177 



Disturbing movements. — After the Huronian formations had attained 

 great thicknesses, the submergence which attended their accumulation 

 was succeeded by a reverse movement, relative or absolute. The 

 beginning of this opposite movement marked the beginning of the 

 end of the Huronian period. The emergence of the land was prob- 

 ably brought about slowly, perhaps as slowly as the submergence 

 which had preceded. Though the border of the sea receded from 

 the position which it had earlier attained, sedimentation continued 



'Sea. Le\/e/ 



di d% d3 



Fig. 61. — Diagrammatic section showing relations which existed around Archean 

 lands during the Huronian period. Huronian sediments (Al) are in process of 

 deposition. They are affected by intrusions and extrusions of lava, d 2 , d 3 , etc. 



along its shores at their successive positions. Figs. 61 and 62 will 

 help to make the relations clear. On the supposition that submerg- 

 ence and emergence of the land were gradual, the duration of Huronian 

 sedimentation must have been greater at some points than at others. 



Fig. 62. — Diagram representing the area shown in Fig. 61 after further sedimentation, 

 intrusion, and deformation. Erosion has also affected the deformed beds. It is to 

 be noted that sedimentation is in progress at the right, and that the sediments now 

 depositing, b, are conformable on the Huronian sediments, //, at the extreme right. 



Thus if Huronian sedimentation was brought about by submergence, 

 it began earlier at a than at b (Fig. 61) and ended later. 



Either as the land emerged, or at some time after emergence, the sedi- 

 mentary beds which had been deposited in horizontal or slightly sloping 

 positions were deformed, being folded in some regions and tilted in 

 others. At the same time it is probable that lava was locally intruded 

 into them and spread out upon their surface. The deformation of 

 the strata and the contemporaneous igneous activity had their effect 



