5 8 SCIENCE PROGRESS. 



replacement of the carbonate of lime by various iron-com- 

 pounds. It is a familiar fact that very many valuable 

 bodies of iron-ores occur not, like most metalliferous 

 deposits, in the form of lodes and veins, but in more or 

 less distinct strata conformable with the associated sedi- 

 mentary rocks. Granting" that, with suitable conditions, 

 iron-compounds, both oxides and carbonate, may be directly 

 formed as stratified deposits, it is still certain that many 

 important cases demand a different explanation. 



The frequent pseudomorphous replacement of calcite by 

 chalybite was pointed out by Bischof thirty years ago, 

 while Blum traced as a further change the conversion of 

 the ferrous carbonate into ferric oxide or hydrate. The 

 chemistry of these processes has been fully discussed by 

 Kimball (9). The geological importance of such trans- 

 formations has become more clearly recognised, however, 

 since the investigations of Judd on the iron-ores of North- 

 amptonshire and Rutland (1875), Sorby on the Cleveland 

 ironstones (1879), and other geologists on various ferrug- 

 inous rocks in this and other countries. The derivation 

 of many deposits of chalybite, haematite, limonite, and 

 magnetite by metasomatic processes from ordinary lime- 

 stones is now placed beyond the possibility of doubt. 



The number of rocks to which this explanation has been 

 applied is greatly increased by the literature of recent years. 

 As instances we mav allude to Lotti's account of the Ter- 



J 



tiary iron-ores of Elba (10) and to the descriptions given 

 by American geologists of numerous examples in the United 

 States. Foerste s contribution on the Clinton (Silurian) 

 iron-ores of the Eastern States presents some points of 

 interest (11). These, like many other stratified iron-ores, 

 are often oolitic ; but in specimens from numerous localities 

 the author finds the grains to show no true oolitic structure, 

 but to be simply rolled and water- worn fragments of bryo- 

 zoans. As a rule the iron-ore occurs as a replacement 

 of the substance of the bryozoans themselves, while the 

 cementing calcite and that which occupied the cells may 

 have suffered the same change, and sometimes the cement 

 has been replaced but the organic fragments preserved as 



