. Bulletin of the Geological Society of America. Geology. 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\'ozoaii
. Bulletin of the Geological Society of America. Geology. 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 fil
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