256 \'tin layl — New Points on the Origin of Dolomite. 



change need not be molecular. In view of this fact compact 

 dolomites showing no shrinkage effects can no longer be 

 regarded as primary. 



Further studies will doubtless show that considerable shrink- 

 age effects produced by dolomitization are not common. It is 

 believed that many vesicular dolomites have resulted from 

 atmospheric leaching long after their formation. 



Petrographic Evidence. 



The microscopic study of many thin sections of dolomitic 

 limestone has not only further amplified and strengthened the 

 field evidence but has also thrown new light upon the details 

 of the process of alteration. By employing microcheinical 

 tests it lias been possible to distinguish between calcite and 

 dolomite in the sections and make clear the most intimate rela- 

 tionships of the two minerals. It should be stated, however, 

 that these tests furnish no reliable guide to the exact amount 

 of magnesia in the rock, for crystals containing less than 25 per 

 cent of MgC0 3 may behave essentially like normal dolomite. 

 But this is to be regarded in truth as a distinct advantage, for 

 alterations of only a slight degree are indicated as well as the 

 more marked ones. 



It must be admitted that if dolomite has a diverse mode of 

 origin the microscope fails to reveal it. Careful examination 

 of every variety of dolomite fails to show any positive evidence 

 in favor of either the primary chemical or the clastic theory of 

 origin. On the other hand, there is abundant evidence in 

 favor of the alteration theory. It is true that certain dolo- 

 mites, whose origin is not certainly known from their field 

 relations, possess an extremely fine and uniform texture, and 

 this feature has in fact led Daly (42) to believe that these repre- 

 sent original chemical precipitates. In order to test the 

 validity of this argument the finest grained dolomite of 

 unknown origin encountered by the writer in these studies was 

 compared with the finest grained dolomite of known secondary 

 origin. For example the Jefferson City dolomite of the Ozark 

 region, whose mode of formation is not definitely known, pos- 

 sesses unusually dense and compact layers which are seen 

 under the microscope to be made up of minute grauules the 

 majority of which are below -003 mm in diameter, some meas- 

 uring only •001 m,n . The granules are suggestive of an original 

 chemical precipitate. The strength of this interpretation is 

 weakened, however, by the fact that a dolomite of known 

 secondary origin has been found in the Middle Devonian of 

 Iowa which is equally as fine-grained. The latter dolomite has 

 resulted from the alteration of a dense, lithographic-like lime- 

 stone with the approximate retention of the original texture. 



