HURONIAN SEDIMENTATION 



327. 



rock and the unconformities between them record the history of 

 tlu- era for this region. 



Proterozoic sedimentation. The surface of the land on which 

 the Proterozoic sediments were deposited was probably comparable 

 to existing land surfaces of crystalline rock which have been long 

 exposed to weathering and other phases of erosion. The topog- 

 raphy was doubtless more or less uneven, and the surface mantled 

 by soil and residual earths and rock debris (mantle rock) which had 

 arisen from the decay of the underlying rock. The general nature 

 of the clastic sediments laid down on such a surface when it became 

 an area of deposition are readily inferred. They were made up 

 chiefly of (i) the disintegrated products already on the surface, (2) 

 the materials worn from the rocks by waves, if the surface was 

 covered by the sea, and (3) river detritus. 



i. One of the first effects of the Proterozoic seas, as they slowly 

 transgressed the land for it is presumed that this transgression 

 was slow was to work over, assort, and re-deposit the loose 

 material on the surface. The coarse sediments were left in the 

 shallow waters, while the fine materials were carried farther from 

 shore, and left in the more quiet waters beyond. Deposits of gravel, 

 sahd, and mud were doubtless being made at the same time in 

 different places, and changes in the position of the shore line, and 

 in the depth of water, brought about, in time, the deposition of fine 

 sediment on coarse, and of coarse sediment on fine. Thus the sedi- 



di 



d 



Fig. apg. Diagrammatic section showing relations which are conceived to 



luve t-xistrd around Arvlu-un lands early in the early Proterozoic. Huronian 

 M-iliments (.11) are in process of deposition. They are affected by intrusions and 

 extrusions of lava, </], (i- 2 , d 3 , etc. 



menlary deposits came to be arranged in beds of different sorts, 

 coarser and finer alternating in vertical section, and grading into 

 each other laterally. 



I At the base of the Proterozoic there is a widespread formation 

 conglomerate (Fig. 290) which appears to be composed of the 



