310 HIE, MONWIRINAUE, QU (Cis QILOGYVG 
Dolomitization—There is a marked difference in lithological 
character between the arenaceo-magnesian beds and the over- 
lying limestones. The former are decidedly magnesian and in 
some cases dolomitic, while the latter are almost entirely calcare- 
ous. Where the brecciated bed is absent there appears to bea 
gradation between the two. The explanations which have 
been proposed to account for dolomitization are: 
(1) Chemical precipitation or sedimentation. 
(2) Leaching of carbonate of lime from a magnesian lime- 
stone subsequent to solidification. 
(3) Replacement of some of the lime by magnesia subse- 
quent to solidification.’ 
From the record of observations it seems probable that all of 
these processes have taken place. With regard to the first view 
it has been shown by Dana? that while “corals themselves con- 
tain very little carbonate of magnesia, magnesia is largely pres- 
ent in some specimens of the rock.” The coral sand and some 
of the compact limestones, however, are said to have very little 
or no magnesia. The conclusion is drawn that the introduction | 
of the magnesian element has taken place (1) in sea waters at 
the ordinary temperature and (2) without the aid of any mineral 
waters except the ocean. The change is thought to have taken 
place in contracting and evaporating lagoons in which the mag- 
nesian and other salts of the ocean were in a concentrated state. 
That dolomite is essentially a littoral deposit has been advocated 
by Calvin,3 while Geikie says that while some dolomites may have 
been formed by chemical precipitation in inland lakes and seas, 
others may have been formed by the action of magnesian salts 
of sea water upon organic limestones already formed.* The 
relations of the magnesian and compact limestones in southeast- 
ern Iowa favor the conclusion that they were formed under 
essentially the same conditions. As the magnesian character 
* Hopkins, T. C.: Geol. Sur., Ark., 1890, Vol. IV., p. 38. 
?Corals and Coral Islands, p. 393. 
3 American Geologist, Vol. I., p. 30, and Vol. II., p. 36. 
4Text-book of Geology, p. 296. 
