EVOLUTION OF LIMESTONE AND DOLOMITE 327 



thick, consisting of conformable, uniformly dense, slightly dis- 

 turbed beds of limestone sharply interstratified with beds of 

 magnesian limestone and dolomite. Analyses taken from various 

 parts of each bed showed that each bed constituted a lithologic 

 unit which often differed sharply from adjacent beds. 



To E. Suess, 1 the sharply defined, thin-bedded intercalations 

 of limestone and dolomite in the Plattenkalk present such powerful 

 evidence of characters which could not have been acquired by 

 metamorphism after their emergence from the sea, that he regards 

 them as direct chemical precipitates. While it may be difficult 

 to agree with Suess that chemical precipitation, in cases like that 

 of the Plattenkalk, is proven, there certainly can be no doubt 

 that it is easier to conceive of such formations as being the direct 

 result of sedimentary processes in the sea, than to believe them to 

 be the result of the differential metamorphism of limestone beds 

 after their emergence from the sea. It is difficult to see how the 

 metamorphism of a succession of conformable limestone beds or 

 formations after their emergence from the sea could be so thor- 

 oughly selective as to result in a succession of beds or formations 

 of unlike composition. Circulating underground water, the most 

 universal agent of the alternation of sediments after their emergence 

 from the sea, is obviously most effective along joints, bedding 

 planes, or other large openings both parallel to and across the 

 bedding, the so-called "trunk channels of circulation" repeatedly 

 emphasized by Van Hise in his" Treatise on Metamorphism." 



Until more direct evidence has been submitted for the origin 

 of dolomite formations through circulating underground water, 

 it seems more reasonable to assume that interstratifications of 

 dolomite and limestone result from primary conditions of sedi- 

 mentation, regardless of what the specific processes of limestone 

 and dolomite building may have been. 



Fineness of grain, peculiar to some dolomites, probably indicative 

 of very little metamorphism since deposition. — Attention is directed 

 to the fineness of grain which some dolomites possess. Daly 2 



1 E. Suess, The Face of the Earth, II, 262. 



2 R. A. Daly, "The Evolution of the Limestones," Bull. Geol. Soc. of America, 

 XX (1909), 167-68. 



