328 EDWARD STEIDTMANN 



finds that the slightly disturbed dolomites of Cambrian and pre- 

 Cambrian age of the forty-ninth parallel section of the Rocky 

 Mountain geosynclinal, aggregating about 7,000 feet in thickness, 

 are singularly monotonous in their fineness of grain, averaging 

 about 0.02 millimeter. Other fine-grained and similarly undis- 

 turbed dolomites mentioned by Daly are those of the Archean 

 at the headwaters of the Priest River, Idaho, the magnesian 

 limestones and dolomites inclosing the chitinous fossil Beltina 

 donai in the Clarke range and the Siyeh and Sheppard siliceous 

 limestones of northwestern Montana, probably of middle Cambrian 

 age. He also quotes Vogt as stating that the finest grained Nor- 

 wegian dolomites average from 0.02 to 0.03 millimeter in diameter. 



Willis and Blackwelder in their studies of the geology of China 

 for the Carnegie Institution describe a number of very fine-grained, 

 dense dolomites of considerable extent and thickness. 



The pre-Cambrian dolomite formation of the Baraboo 1 syn- 

 clinal is singularly fine grained. The Niagara limestone of Wis- 

 consin, a dolomitic formation, is generally a very even-textured, 

 fine-grained, and compact rock. 



The generalization may be deduced from the facts cited that 

 many undisturbed or only slightly disturbed dolomites are exceed- 

 ingly fine grained and compact. There does not appear to be any 

 evidence that this fineness of grain is the result of granulation. 

 Furthermore, the relative mobility and regenerative power of the 

 carbonates is very great as compared with the other minerals. 

 Coarse-grained, even-textured 2 marbles showing no strain effects 

 are found interbedded with banded gneisses with marked strain 

 effects. Unless conditions are peculiarly unfavorable for recrystal- 

 lization, the metamorphism of dolomites would probably result 

 in coarser grain. The fineness of grain and compactness of many 

 dolomites may therefore be interpreted as showing that they were 

 dolomite before they emerged from the sea. 



Many dolomite formations lack fossils. — The opinion is reflected 

 from geologic literature that fossils are less common in dolomite 



1 S. Weidman, "Baraboo Iron-bearing District," Wisconsin Survey Bull., No. 13 

 (1904), p. 67. 



2 C. R. Van Hise, "Treatise on Metamorphism," U.S.G.S. Mon. 47, p. 738. 



