76o PRINCIPLES OF STRATIGRAPHY 



True, not all dolomites are of secondary origin, some being no doubt 

 deposited as dolomite rock in the beginning. Among dolomites of 

 secondary origin we may distinguish those derived by the clastation 

 and redeposition of older dolomites and those due to the replace- 

 ment of limestones. Only the latter class belongs here, but the 

 dolomites of clastic origin deserve brief attention. Here belong 

 the many well-bedded, fine and uniformly grained rocks with few 

 or no fossils which abound in many Palaeozoic and later forma- 

 tions. As a typical example may be mentioned the Monroe (Upper 

 Siluric) dolomites of Michigan, Ohio and Ontario, which have 

 most probably been derived by the destruction of the older Niagaran 

 dolomites and deposited as dolomitic sand and mud. This is prob- 

 ably the origin of most of the fine-grained, well-bedded dolomites 

 which, from the fact that they contain scattered fossils, are seen 

 not to be the product of alteration of limestone. 



When limestones and dolomites are found interstratified, the 

 successive beds being sharply dififerentiated from one another, this 

 seems to be most satisfactorily explained as a primary difference in 

 the materials deposited. Suess (31, 11:262) regards this altera- 

 tion in the Plattenkalke as due to alternate chemical precipitation 

 of dolomites and limestones, but in practically all rocks of this 

 type a clastic origin of the deposit must be postulated. In other 

 words, the beds are calcilutytes. some of them pure, others mag- 

 nesian, the mud being derived alternately from calcareous and mag- 

 nesian sources. Or. again, the limestones may be of organic origin, 

 while the enclosed dolomites are of terrigenous origin, being derived 

 from the erosion of dolomites forming a portion of the land, and 

 such alternation would have no more significance than alternations 

 of limestones (of thalassigenous origin) and shales (of terrigenous 

 origin). The possibility of secondary separation of a mixture of 

 lime and dolomite grains by agitation of the water and the unequal 

 settling according to specific gravity must not be overlooked. 



Secondary dolomites due to diagenetic alteration processes may 

 originate either before or after the original limestones are raised 

 above the sea-level. (Steidtmann-30.) Such alteration may be 

 primarily a process of leaching, either under the sea by sea water 

 or by the ground water circulating through the upper zones of the 

 earth's surface. By leaching out of the lime the proportion of the 

 original magnesian content is greatly increased. Such differential 

 leaching is due to the fact that calcium carbonate is several times 

 as soluble as magnesium carbonate as first shown by Bischoff. When 

 it occurs, one result is the rendering porous of the altered rock, 

 which, if under pressure, may actually collapse. The process of 



