166 GEOLOGY. 



represented by the unconformities are unrecorded, so far as exposed 

 strata are concerned, though they are recorded by the unconformities 

 themselves. It is not impossible that the interval represented by uncon- 

 formity in one region is represented by known strata in another; but 

 in the absence of fossils, this relation is difficult of determination. 

 Whether the stratigraphic records made in these intervals are ever 

 identified or not, they must be taken into account in the conception 

 of the Proterozoic era as a whole. This era therefore includes the 

 time when at least three thick systems of strata were deposited, and 

 two intervening intervals, represented by unconformities, but not 

 now known to be recorded by accessible strata. Nor is this all; the 

 unconformities between the Proterozoic formations and the Archean, 

 on the one hand, and between the Proterozoic and the Paleozoic, on 

 the other, are to be taken into account. Some portion of the time 

 represented by each of these two great unconformities is to be assigned 

 to the Proterozoic. 



Proterozoic Sedimentation. 



General considerations.— The stratigraphic relations of the sedimentary 

 Proterozoic formations to the Archean show that they were laid down 

 on the bed of a sea, or other lodgment area, which, at the points where 

 the formations are known, was advancing upon the land. At the 

 time of the advance of the Proterozoic seas (taken as representative of 

 all lodgment areas) upon the lands of Archean rock, the surface of 

 the latter was probably comparable to existing land surfaces of crys- 

 talline rock which have been long exposed to weathering, and other 

 phases of erosion. The topography was doubtless more or less uneven, 

 and the surface mantled by soil and residual earths (mantle rock, 

 regolith, geest) which had arisen from the decomposition of the under- 

 lying rock (Vol. I, p. 41). Large and small masses of rock, more resist- 

 ant than their surroundings, probably remained undecomposed, or but 

 partially decomposed, in the earths which represented the products of 

 more complete decay. 



The general nature of the clastic sediments laid down on the eroded 

 surface when it was converted into an area of deposition may be 

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

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

 rocks by the waves, and (3) of river detritus. 



