114 PROCEEDINGS or THE BALTIMORE MEETING 



near Chewacla there is strong evidence for believing tlie rocks to be of Paleo- 

 zoic age. About two miles southeast from Opelika there occurs a ridge of what 

 appears to be Knox chert associated with mica schist and gneisses. About a 

 quarter of a mile west of this chert ridge there is a ten-foot bed of low-grade 

 iron ore which strongly resembles the low-grade iron ores of the Clinton of the 

 Birmingham region. About five miles southeast of Opelika and three miles 

 southeast of Chewacla there is a long narrow strip of dolomitic marble which 

 has on either side of it narrow strips of what appears to be Knox chert. To 

 all appearances, this belt of dolomite and chert is infolded and overturned 

 toward the west. 



Presented by title in the absence of tlie author. 



COXTRIBUTIOX TO THE ORIGIX OF DOLOMITE 

 BY W. A. TARR 



(Abstract) . 



Field studies carried on in the Ozark area of Missouri during recent years 

 have furnished evidence bearing on the time and place of dolomitization. The 

 evidence favors the view advocated by Steidtmann that the magnesium car- 

 bonate is added to the calcium carbonate on the sea-bottom, probably immedi- 

 ately after the deposition of the calcium carbonate. The almost absolute purity 

 of the dolomites in the Ozark area is favorable to his view that gradations do 

 not exist between calcite and dolomite and favors the suggestion made below. 

 Analyses of the dolomites show that they contain up to .85 per cent iron oxide, 

 mostly FeO, and always much more iron than aluminum, thus accounting for 

 the remarkably deep red soils over these dolomites. The dolomites in this area 

 are shallow-water deposits in large part, showing all its usual features of such 

 deposits. 



Further studies made by mapping the dolomite areas according to the geo- 

 logic period during which the dolomite was deposited have shown that these 

 areas were occupied by broad, shallow, inclosed seas, in which there might have 

 been a concentration of magnesium and other salts above that of normal sea- 

 water. This concentration of salines and the probable increase in the amount 

 of magnesium salts would greatly favor the rapid formation of dolomite. This 

 would explain why practically pure limestones, with only 1 to 3 per cent of 

 magnesium carbonate, are found interbedded with pure dolomites. The fresh- 

 ening of the water in the inclosed seas by influxes of normal sea-water would 

 prevent the formation of dolomite until the concentration in salinity or in the 

 magnesium content had again been brought about. Not a single occurrence 

 has been found in the Cambrian and Ordovician dolomites of Missouri where 

 the limestone grades into dolomite. 



The work done to the present favors the marine origin of dolomite and the 

 view that deposition occurred in shallow, inclosed seas where increased salinity 

 or concentration of magnesium salts might occur. 



Presented bv title in the absence of the author. 



