THE PROTEROZOIC ERA. 167 



(1) 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 found 

 on the surface. The larger masses of rock suffered relatively little 

 transportation and wear; the sand and small bits of rock were rolled 

 along the bottom and deposited in relatively shallow water, while the 

 finer materials were carried out from the shore and deposited in the 

 more quiet waters beyond (Vol. I, p. 369 et seq.). Deposits of gravel, 

 sand and mud were doubtless being made at the same time at different 

 distances from the shore, and from the gravel on the one hand, to the 

 finest mud on the other, there were all possible gradations (Fig. 50). 



Changes in the position of the shore line, and changes in the depth 

 of water incident to the sinking of the land, may have brought about 

 the deposition of fine sediment on beds of coarse material deposited at 

 an earlier time (Fig. 50) when the water where they occur was shallow, 

 and a return of shallow water, - as by the rise or aggradation of the 

 sea-bottom, may have caused coarse sediments to succeed the fine 

 in vertical section. In such ways the sedimentary deposits came to 

 be arranged in layers, coarser and finer alternating with one another 

 in veitical section, and grading into one another laterally. 



If, at all stages of the sinking which brought land composed of Ar- 

 chean rock below the Proterozoic seas, the waters found coarse and 

 fine materials on the surface they transgressed, and if at all stages the 

 finer parts were carried out from the shores, there should be a wide- 

 spread deposit of coarse material (gravel) derived from the under- 

 lying rock at the base of the sedimentary series (Fig. 50). Such a 

 formation is known as a basal conglomerate, and is often one of the 

 best indices of an unconformity. All parts of a bed of basal conglom- 

 erate were not necessarily contemporaneous in origin. Thus the coarse 

 material at a (Fig. 50) is older than that at Al, with which it is 

 laterally continuous. 



It is to be especially noted that the loose residual materials which 

 the advancing Proterozoic seas found upon the surface which they 

 transgressed had arisen chiefly by rock decay, and that they were 

 therefore essentially unlike their parent formation in average compo- 

 sition (Vol. I, p. 422). 



(2) Besides working over the regolith, the waves doubtless attacked 

 the solid rock wherever exposures were favorable, just as waves here 



