442 



E. STEIDTMANN ORIGIN OF DOLOMITE 



ill turn by iseveral feet of uearl}' pure limestone. In tlie mixed beds the 

 dolomitized masses are dome-shaped, abont 2 feet higli and 3 feet in 

 diameter at the base. The bedding cuts across the domes as shown in 

 figure 2. They are not in any way related to shaly seams or to joints, 

 but are primary structures. TJlrich^*^ stated that in the domes the chief 

 fossils dolomitized were the pelecypods, and that the pelecypods were 

 largely in the domes, while originally calcific tests, such as the br\'ozoaiis, 

 crinoids, and brachiopods, were much more abundant in tlie limestone 

 phases. Since the pelecypods are nearly all aragonite originally, it would 

 seem tliat mouud-sliaped colonies of pelecypods had'been more susceptible 

 to dolomitization than the adjacent calcific deposits. 



THIN BEDOeO 

 LIME STONE 



H£AVYO£DDeD 

 DOLOMITE 



FiGUiJE 2. — Diafjraiiiiiiatic Vicic of Dolomite Domes 



The diagram is based on the dolomite domes in the limestone beds in the Galena- 

 Trenton formation northeast of Sun Prairie, Wisconsin. The domes have tlie same bed- 

 ding planes as the adjacent limestones and are not related to fissures or shaly seams. 

 They contain numerous dolomitized pelecypods, whereas the limestone parts are rich in 

 bryozoa, brachiopods, and crinoids. 



Relations of dolomite grains to each other and to calcite grains as 

 slmivn by stains. — The dolomite grains of beds lacking primary calcite 

 varied in size from less than .01 mm. up to .15 mm. or more. A few 

 had grains .6 mm. in width. They were, as a rule, anhedral when 

 l)ounded by their own kind. The pattern in most cases is a close httiug, 

 interlocking mosaic, apparently water-tight, judging from the lack oi 

 oxidation of the FeO in the dolomite and the occasional presence of very 

 fresh triclinic feldspars. Some of the Lower Magnesian and Galena dolo- 

 mite samples, however, are composed of loosely fitting grains fringed w itli 

 limonite films. Evidenly they are quite pervious. Plate 27 shows an 

 unusual type, a dolomitic phase of the Franconia sandstone. The dolo- 

 mite rhombs are .6 mm. in diameter and include quartz grains, showing 

 that the rhombs grew in the sediment. The matrix is a brown, opaque 

 mass full of anguhir quartz grains. 



i« Personal statement. 



