THE PROTEROZOIC ERA 451 



topography was doubtless more or less uneven, and the surface 

 mantled by soil and residual earths (mantle rock) which had arisen 

 from the decay of the underlying rock. Large and small masses 

 of rock, more resistant than their surroundings, probably remained 

 undecayed, or but partially decayed, in the earths which repre- 

 sented the products of more complete decomposition. 



The general nature of the clastic sediments laid down on such 

 a surface when it became an area of deposition may be readily 

 inferred. They were made up chiefly of (1) the disintegrated 

 products already on the surface, (2) the materials worn from the 

 rocks by the waves, if the surface was covered by the sea, and 

 (3) river detritus. 



(1) One of the first effects of the Proterozoic seas, as they 

 slowly transgressed the land for it is presumed that this trans- 

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

 loose material found on the surface. The larger masses of rock 

 suffered little transportation and wear, the sand and small bits 

 of rock were rolled along the bottom and deposited in relatively 

 shallow water, while the fine materials were carried out from the 

 shore and deposited in the more quiet waters beyond. 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. Changes in the position of the shore line, and changes 

 in the depth of water incident to the sinking, of the land or the sea 

 bottom, brought about the deposition of fine sediment on coarse, 

 and of coarse sediment on fine. Thus the sedimentary deposits 

 came to be arranged in beds of different sorts, coarser and finer 

 alternating in vertical section, and grading into each other laterally. 



As the land of Archean rock was submerged by the Proterozoic 

 seas, the coarse materials of the mantle rock were left upon the 

 surface in many places, with little movement and little wear. At 

 the base of the Proterozoic there is therefore often a wide-spread 

 deposit of coarse material (gravel, etc.) derived from the under- 

 lying rock (Fig. 345). Such a formation is known as a basal con- 

 glomerate, and is often one of the best indices of an unconformity. 



If the climatic conditions were like those of the present, the 



