THE PROTEROZOIC ERA. 169 



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 presumably over the whole of the ocean bot- 

 tom. The same may have been true from the beginning of the Proterozoic 

 era, for the beginnings of life go even farther back. Both from extra- 

 terrestrial sources, and by precipitation from solution, further addi- 

 tions may have been made to the sediments accumulating on the sea 

 bottom. Sedimentation, even if slow, was therefore doubtless in 

 progress in the deeper parts of the Proterozoic seas as well as near the 

 shores. These general considerations prepare us for the study of the 

 sedimentary rocks now exposed. 



The sediments and their interpretation. — The sedimentary beds of 

 the Proterozoic consist of conglomerates (Vol. I, p. 468), sandstones, 

 (Vol. I, p. 422), shales (Vol. I, p. 473), and limestones (Vol. I, p. 378), 

 or of their metamorphic equivalents. Before being cemented or other- 

 wise solidified into firm rock, their materials were gravel, sand, mud, 

 etc. The manner in which such materials are now being derived 

 from preA^ious formations is set forth in Volume I, pp. 110-114, 332- 

 338, and 420-426; the manner in which they are transported by streams, 

 waves, and ocean currents is explained on pp. 115-119, 354, and 364 

 of the same volume; and the way in which they are assorted, distributed, 

 and deposited is discussed on pp. 177-204 and 355-386. The facts 

 and principles there set forth should be well in mind before the history 

 of this and succeeding periods is studied. In so far as the Proterozoic 

 sediments had the same characters as the modern sediments, it is 

 inferred that they were formed in similar ways. 



At the base of the Proterozoic, and at the base of its several systems, 

 there are sometimes found beds of basal conglomerate, which, when 

 traced laterally, are found to abut against steep slopes (cliffs) of the 

 underlying formation. This is exactly the relation which the gravel 

 and shingle of existing beaches sustain to the shore rock. It is therefore 

 inferred that these ancient conglomerates represent ancient shore 

 tracts. As the beds which overlie the basal conglomerates are usually 

 of finer materials, the shore is inferred to have advanced upon the 

 land as sedimentation proceeded, and thus to have given opportunity 

 for the deposition of off-shore material over the coarse shore deposits. 



