762 PRINCIPLES OF STRATIGRAPHY 



offer better access to waters bearing magnesium, through the agency 

 of which the replacement is brought about. 



The replacement of calcite by dolomite involves a contraction of 

 12.30%. Dolomites due to alteration will thus show a high degree 

 of porosity unless they have been subjected to compression during 

 orogenesis. Such porosity is shown in the early Palaeozoic dolo- 

 mites of the Mississippi Valley, and also in the Siluric and the 

 Devonic dolomites of Michigan, Ohio and Canada. When the rock 

 is under pressure, as in the zone of anamorphism, mashings and 

 recrystallizations close the openings. It is also highly probable that 

 pressure promotes dolomitization, since this means a decreasing 

 volume, a result favored by pressure. 



IV. Replacement of Limestones by Silica, Iron Oxide, Etc. 



Metasomatic replacement of limestone by silica is a familiar 

 phenomenon. In most cases the replacement affects chiefly certain 

 parts of the limestones which by their structure seem best suited to 

 such replacement. Such are the shells, corals and other organic 

 remains embedded in Palaeozoic or younger limestones where the 

 enclosing matrix generally remains unaffected, though the fossil 

 may be completely replaced. Oolitic limestones also suffer replace- 

 ment by silica and in them often the steps of replacement are shown 

 by the decrease in lime and the increase in silica. A mass of such 

 siliceous oolite occurs in the lower Palaeozoic rocks of central 

 Pennsylvania, where it covers an area of about 40 square miles, 

 with scattered extensions over a much wider area. Locally the 

 oolite passes into chert. These siliceous oolites have been regarded 

 as originating in rising hot springs containing silica in solution 

 (Wieland-36:P(5i'), but others (AIoore-23; Brown) hold that 

 they represent replacements of originally calcareous oolites. This 

 explanation is fully borne out by the analysis, and the incomplete- 

 ness of the replacement in many cases. ( See also Ziegler-37.) 



Replacement of limestone by iron oxide is also a frequent occur- 

 rence. In the basal Siluric beds of Wisconsin, oolites, which from 

 their character and appearance were most probably calcareous in 

 the first place, have been changed to iron oxide (hematite). The 

 so-called Clinton iron ore seems to be an example of metasomatic 

 replacement of limestones, for here the organic fragments (brachi- 

 opod shells, Bryozoa, etc.) are replaced by iron oxide. In the Gen- 

 esee gorge at Rochester the steps in replacement could formerly be 

 observed, these being shown by the progressive increase in iron 



