GEOLOGICAL IMPORTANCE OF SEDIMENTATION 567 



of subaerial origin. The two regions are unusually favorable for 

 study in this particular, since, as previously noted, the deposits 

 have been relatively little disturbed by later earth movements and 

 the original sedimentary record has not been obliterated through 

 the processes of metamorphism. 



A general conclusion should be founded on a far wider study of 

 occurrences, but such would run beyond the hmits of this paper. It 

 may be noted in passing, however, that the Montana region shows 

 an unusual proportion of carbonate rocks, while the pre- Cambrian 

 deposits over the world as a whole apparently are characterized by 

 minor amounts of carbonates, rocks whose presence in notable 

 proportions are usually the surest indication of truly marine con- 

 ditions. Such a poverty in limestones may in a small measure be 

 accounted for by a possible dominance of disintegration over decom- 

 position in the erosion of those times, the hme thus in part not being 

 set free and the disintegrated products giving rise upon metamor- 

 phism to a large proportion of gneisses, graywackes, and feldspathic 

 schists, instead of quartzites, argiUites and marbles. 



Highly sihcious rocks are, however, not uncommon, and the 

 question arises as to where the corresponding quantities of salt, 

 gypsum, and dolomitic hmestones are to be found. In the long 

 time elapsing since their origin these might have been completely 

 leached out by subterranean waters, as Rutley has shown, ' if they 

 had remained near the surface in the zone of circulating waters. 

 But the pre- Cambrian rocks are usually highly metamorphic and 

 have been buried deeply in the zone of anamorphism during a con- 

 siderable proportion of their existence, so that such an explanation 

 can hardly apply to them in very much greater measure than to the 

 Eopaleozoic limestones which remain in such abundance. 



The bulk of the salt is doubtless still in solution in the sea and 

 is a measure of the volume of erosion in those early ages in addition 

 to that of later times. The corresponding dolomites, however, 

 since they are apparently not found in proportionate abundance 

 upon the continents must presumably repose within the limits of 

 the present ocean basins. 



' "On the Dwindling and Disappearance of Limestones," Quarterly Journa 

 Geological Society, Vol. XLIX (1893), p. 372. 



