328 PROTEROZOIC ERA 



coarse parts of the mantle rock which were on the surface when the 

 Proterozoic seas transgressed the lands of Archean rock. Such a 

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

 indices of an unconformity. 



2. Besides working over the decayed rock, the waves doubtless 

 attacked the solid rock wherever exposures were favorable. The 

 sediments thus acquired resembled the parent formation in average 

 composition, and are thus distinguished from those of the preceding 

 class, which were the products of rock decay. 



3. Streams descending from the land must have brought down 

 gravel, sand, and mud. The larger part of the river-borne detritus 

 was probably decomposed rock, but a smaller part was doubtless 

 derived by the mechanical action of running water on undecayed 

 rock. Once in the sea, these several sorts of detritus were mingled. 



Since some of the constituents (especially alkalies and alkaline 

 earths) of the Archean rock dissolved during the processes of de- 

 composition probably remained in solution in the sea-water, it is 

 thought that the clastic sediments were more siliceous than the 

 rock from which they were derived. 



The sorting power of moving water takes account of the physical 

 characteristics of the material handled, and not of their chemical 

 constitution; but in the decomposition of Archean rock, the quartz 

 remaining in the residual mantle was generally in larger particles 

 than the clayey matter derived from the silicates, and under the 

 sorting influence of the waves the quartz grains (sand) were more or 

 less completely separated from the clayey parts (mud). Thus 

 materials which were unlike chemically were separated from one 

 another because they were unlike physically. If the Proterozoic 

 seas had abundant life which secreted calcium carbonate, or if their 

 waters anywhere became overcharged with calcium carbonate, lime- 

 stone might have been formed. 



Extent. Sediments accumulated in the Proterozoic era are 

 known in limited areas only, but doubtless they were very wide- 

 spread. Water-borne and wind-blown sediment must have reached 

 all parts of the sea, and the life of the salt water probably made 

 deposits over the whole of the ocean bottom. Some sediments, too, 

 must have been left on land, as at all other stages of the earth's 

 history since sedimentation began. 



The exposed formations. The sedimentary beds of the Protero- 

 zoic consist of conglomerates, sandstones, shales, and limestones, 



