446 E. 8TE1DTMANN ORIGIN OF DOLOxMlTE 



Trenton forma tiuji mI Etna, A\'isc'on8in, shows a mat of cTliiidrical calcite 

 iiulividiials averaging al)out .5 inch in length and .2 incli in diameter — 

 pronounced algal secretions hy Dr. E. 0. L'lrich,^'^ which are partially 

 dolomitized on the surface and along cracks. Plate 25 shows a cross- 

 section of one of tliese masses. Tlie matrix in which they are emhedded 

 is a granular dolomite. Where the algal growths are absent the bed con- 

 sists wholly of the dolomite. It is ]iot at all true, however, that lime- 

 stones in general Jiave escaped dolomitization because of comi)actness 

 (see tables 1 and 2, page -ioS). 



Diagram 1 shows that, as a rule, the beds of mixed calcite and dolo- 

 mite have more insoluble constituents than the pure limestones and 

 dolomites. The significance of this is doubtful. Since muds are more 

 common in the carbonates than sands, it seems likely that dolomitization 

 was checked by the sealing effect of the muds. 



DEPTH OF MATER 



All the Paleozoic dolomites of AYi scon sin were laid down in shallot' 

 water. Tiiis is evidenced Ijy reef structures, ripple-marks, conglomerates, 

 cross-bedding, sun cracks, and association with sands and muds. Most 

 dolomites appear to have been deposited in shallow seas. It does not 

 follow, however, that deep A\'aters pi'ohil)it the development of dolomite. 

 It is equally true that most marijie limestones exposed on the lands are 

 of shallow- water origin. 



TEMPERATURE OF MATER 



Tlie type of life preser\ed iji the dolomites studied proves tliat they 

 were formed in warm seas. In fact, no proof could be found anywhere 

 of dolomites having formed under distinctly frigid conditions. Ex])eri- 

 ments indicate tliat under certain conditions dolomitization is stimulated 

 by temperatures abo^•e 30° C. Deposition in ^varm seas is not peculiar 

 to dolomites, but is equally characteristic of most limestones. 



COMPOS J TIOX OF SEA-WATER 



Special emphasis is given to the FeO content of dolomites which was 

 brought out by staining methods and to its absence in the primary calcite 

 grains of about 300 sam])les mostly from Wisconsin, but including some 

 from Wyoming and AVinnipeg. Ye'm calcite examined usually showed 

 FeO by staining. Slialy streaks in limestones and dolomites unless 

 strongly oxidized also reacted for FeO. The difference in FeO content 



1° Porsonal conimuDication. 



