THE PROTEROZOIC ERA. 171 



it. Some of the sandstones contain abundant grains of feldspar and 

 other silicate minerals, from which it is inferred that the conditions 

 under which their materials were derived did not permit mature decom- 

 position. As the more notable sandstones (wackes) of this type are 

 associated with lava beds, it is a consistent inference that as a 

 result of the volcanic fragmentation that accompanied the outflows, 

 and the naked surfaces that succeeded them, the ratio of surface wash to 

 decomposition was high, and that fragmental material was borne 

 away to the depositing grounds before being completely decomposed. 



To account for the thick series of sediments which exist, it seems 

 necessary to suppose that there was in progress, in some places at 

 least, a slow, quiet warping of the surface which lowered the lodgment 

 basins about as fast as they were filled, and perhaps at the same time 

 raised the areas from which the supply of material came, though this 

 last inference is not a necessary one (Vol. I, pp. 540-541). 



By such comparisons and inferences, it is concluded that the general 

 conditions of the Proterozoic times were more like, than unlike, those 

 of the present time, though the points of difference were considerable, 

 especially the greater prevalence of igneous action. Both the resem- 

 blances and the differences will be more apparent after the details of 

 the formations are studied. 



Geographic Relations of the Exposed Proterozoic to the Exposed Archean. 



Proterozoic rocks appear at the surface in many parts of North 

 America, but in few regions have they been clearly separated from 

 the Archean, and in very few have their subdivisions been worked out 

 so as to permit of their separate delineation on maps. Fig. 38 shows 

 the area where rocks of known Proterozoic age lie at the surface, as 

 well as areas where Proterozoic rocks have not been differentiated 

 from the Archean. Where their relations have been made out, the 

 Proterozoic rocks are found to be associated in many places, but not 

 in all, with areas of exposed Archean. Where Proterozoic and 

 Archean rocks are associated at the surface, the former are often found 

 about the borders or along the sides of areas of the latter, or sometimes 

 in narrow belts, or in isolated patches within them. These various 

 relations are shown on the map (Fig. 38), and also by Figs. 51 to 56. 



That the Proterozoic formations should be exposed most commonly 

 about the borders of the Archean is made clear by Fig. 51, which shows, 



