96 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 64 



formations of Paleozoic age have been deposited under conditions which 

 are notably different, as regards salinity, temperature, or depth, from those 

 under which non-magnesian formations, either argillaceous or calcareous, 

 have been laid down. Chemical geologists are almost unanimously agreed that 

 in general the dolomitization of limestone is a secondary process, and the 

 paleontological evidence, so far as it is available, seems to substantiate that 

 view. Formations now dolomite were in all probability originally deposited 

 as limestones, and have been altered to dolomites since their original deposition, 

 while other beds entirely similar in original condition have not been modi- 

 fied, but persist to the present time as true limestones. 



Thus far my observations have led to practically the same con- 

 clusion for the Algonkian and Cambrian magnesian limestones. 

 There was in all probability a small percentage of magnesian salts 

 deposited through the agency of Bacteria and Algge in the epiconti- 

 nental Algonkian sediments, but it was a secondary process that 

 produced the limestones with a high percentage of magnesia. 



Definition of magnesian limestone and dolomite. — In response to 

 my request Mr. E. F. Burchard, of the United States Geological 

 Survey, sent the following note on magnesian limestone and dolo- 

 mite. I think it would be well if some such classification of the 

 magnesian rocks could be generally accepted, as the term dolomite 

 has been very loosely used in geologic literature : 



Magnesian limestone. — Magnesian limestone is limestone containing mag- 

 nesium carbonate in any quantity up to 45.65 per cent. The majority of mag- 

 nesian limestones carry either a small percentage or a high percentage of 

 magnesium carbonate, although there are many deposits that are intermediate 

 in composition. 



Dolomite. — Dolomite is a mineral composed of the double carbonate of 

 calcium and magnesium (CaCOs -IMgCOs). It contains 54.35 per cent 

 CaCOs and 45.65 per cent MgCOs. In practice, magnesian limestone, con- 

 taining 20 per cent or more of magnesium carbonate generally has been 

 called dolomite, but it would be preferable if magnesian limestone could be 

 distinguished as " low magnesian " and " high magnesian," restricting the term 

 dolomite to rock containing nearly, if not quite, the theoretical quantity of 

 magnesium carbonate necessary to combine with the calcium carbonate in the 

 proportions given above, or in the ratio of 1:1.19. The mineral dolomite 

 in places form rock masses, in which the crystals of dolomite can be dis- 

 tinguished. In some rocks these crystals make up a large proportion of 

 the beds, and on weathering, the rock crumbles to a sand composed of 

 dolomite crystals. Rock and sand of this character are common in southwest 

 Wisconsin near the junction of Wisconsin and Mississippi rivers. The texture 

 of magnesian limestone and so-called dolomite is commonly rather rough and 

 moderately coarse on weathered surfaces. 



In the formation of magnesian limestone and dolomite, magnesium carbo- 

 nate is believed to have replaced calcium carbonate, either while the beds were 

 being deposited in the sea, or after the beds become part of the land surface. 



