22 KEMP'S ORE DEPOSITS. 



general shrinkage of eleven or twelve per cent. In any extended 

 thickness of strata this would cause vast shattering and porosity. 

 As an illustration of its results, the following analyses of normal, 

 unchanged Trenton limestone of Ohio, and of well drillings from 

 the porous, gas-bearing, dolomitized portions of the same, are 

 given. They are taken from a paper by Edward Orton. (Amer. 

 Manuf. and Iron World, Pittsburg, Dec. 2, 1887.) 



CaC0 3 . MgC0 8 . Fe 2 3 .Al 2 3 . SiO 2 . 



Unchanged Trenton limestone... 79. 30 0.92 7.00 12.00 



...82.36 1.67 0.58 12.34 



Dolomitized " " ...53.50 43.50 1.25 1.70 



...51.78 36.80 



1.02.14. Late studies in ore deposits by Posepny, Curtis, and 

 Emmons indicate also that solutions of metallic ores may effect an 

 interchange of their contents with the carbonate of calcium or 

 magnesium, in limestones and dolomites, leaving an ore body in 

 place of the rocks. This change is effected molecule by molecule, 

 and is spoken of as a metasomatic interchange or replacement. 

 (See Example 30.) By "metasomatic" is meant an interchange 

 of substance without, as in pseudomorphs, an imitation of form. 

 Alteration of the metallic ores may follow and occasion cavities 

 from shrinkage. (See Example 36, and Curtis, on Eureka, Nev., 

 Monograph VIII., IT. S. Geol Survey, Chap. VIII.) 



