THE PROTEROZOIC ERA 453 



their chemical constitution; but in the decomposition of Archean 

 rock, the quartz remaining in the residual mantle -was generally 

 in larger particles than the aluminous products of the decomposition 

 of the silicates, and under the sorting influence of the waves the 

 quartz grains (sand) were more or less completely separated from 

 the aluminous particles (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 be- 

 came overcharged with lime carbonate, limestone might have been 

 formed. 



Extent. While the sediments accumulated in the Proterozoic era 

 are known in limited areas only, it is to be borne in mind that Pro- 

 terozoic sediments were in reality more wide-spread than the seas 

 of the time; for though coarse material from the land is not usually 

 washed out far from the shore, material fine enough to be carried 

 in suspension may be transported great distances, and small 

 amounts of dust are constantly being blown from the land to all 

 parts of the sea. In later 'geologic time, the life of the sea has 

 occasioned considerable deposits even far from the land, and pre- 

 sumably over the whole of the ocean bottom. The same may have 

 been true from the beginning of the Proterozoic era, for the begin- 

 nings of life go even farther back. From extra-terrestrial sources, 

 and by precipitation from solution, further additions may have 

 been made to the sediments accumulating on the sea bottom. 

 Sedimentation, even if slow, was therefore doubtless in progress 

 everywhere in the Preterozoic seas, and on some parts of the land 

 as well. 



The exposed formations. The sedimentary beds of the Proter- 

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

 or of their metamorphic equivalents. Before being cemented or 

 otherwise solidified into firm rock, their materials were gravel, 

 sand, mud, etc. The manner in which such materials are derived 

 from older formations and transported to places of deposition, has 

 been explained. 



Basal conglomerate is of rather common occurrence at the base 

 of the several systems of the Proterozoic, while the overlying beds 



