SIJMMARY 449 



of limestone and dolomite; the partial to complete replacement of lime 

 carbonate skeletons by dolomite, and by cases of local dolomitization adja- 

 cent to or ^vithin pervions marine structure, such as worm borings, shell 

 cavities^ etcetera. 



Dolomite grains in contact with their own kind are generally anhedral 

 and at best only roughly rhombohedral. Those in contact with calcite 

 grains were uiuler all conditions found to be rhombohedral, whereas all 

 calcite grains were anhedral and smaller tban the dolomite rhombs with 

 which they occur. In no case examined by the writer did dolomite grains 

 contain undigested calcite residuals. The sharp borders l)etween calcite 

 and dolomite grains aud tlie knife-edge contacts of dolomite invaders in 

 casts ]:»rove that certain replacements of calcite by dolomite were accom- 

 ])lished without volume cliange. No evidence could be found to show 

 that it ever takes place witli Aolume change. 



Convincing evidence could iiot l^e found for dolomitization by recrys- 

 tallization of MgO-bearing lime carbonate skeletons. It is suspected that 

 it takes place from the fact that many modern lime carbonate skeletons 

 contain up to 1-1 per cent ]\[g('0., without showing dolomite. 



Crystallization from the waters iu tlie ooze is suggested by the inclu- 

 sion of dolomite rh(mdx)hedra in dense, fine-grained masses of calcite, 

 l)ut rejdacement of lime carbonate before consolidation explains this re- 

 lationshij) equally well. In recently u])lifted coral reefs the ])resence of 

 dolomite crystals in druses a-n([ vugs sliows that crystallization from sea- 

 waters withiiL the sediments does occur under certain conditions. 



Proof of direct precipitation of dolomite from the sea was not found. 

 The fine grain of some dolomites, a grain much finer tlian that of the 

 dolomites which are clearly due to re]olacement, and the similarity in 

 grain of the fine-grained dolomites to modern bacterially precipitated 

 lime carbonate oozes suggest strongly that these (ine-grained dolomites 

 are cliemical precipitates. If they arc lu-ecipitates they probably were 

 thrown out of the lower strata of water, since they contain FeO. Only 

 the lower strata of water could be cx|)e('led to be reducing in action and 

 thus permit the precipitation of FeO. 



CONDITION OF AfAUTXE })OrA)M ITTZATION 



Marine dolomites were mostly de))Osited in warm, shallow seas; but this 

 seems to be equally true of most limestones. A minoritv^-were laid down 

 in very saline waters. The salinity of the open season which most of 

 them were formed was not sufficiently dift'erent from that of limestone- 

 depositing seas to impress itself on the fossils. Ex])eriments show that 

 increase in salinitv stimulates dolomitization. 



