THE EARLIER FORMS OF LIFE. 259 



Huronian ores of Lake Superior have the same composition as the 

 Silurian deposits. Lastly, the Laurentian magnetites constitute the 

 other extreme of the ferruginous series. Both the water and a part 

 of the oxygen have disappeared, leaving a compound richer in metal, 

 and therefore more highly prized by the smelter. The application of 

 a gentle, continuous heat is adequate to explain the change of the 

 limonites into hematites and magnetites. 



The process of change may be seen in the manufacture of common 

 bi'icks, or the purification of quartz for the production of glass. The 

 blue clay becomes red when burned, because it parts with its water 

 of composition ; and likewise the small percentage of hematite in the 

 quartz becomes magnetic on the application of heat, and, after pulver- 

 ization, has the iron removed by magnets, soHhat the silica-flour may 

 be perfectly pure, and not impart a green tint to the glass. It is not 

 maintained that the native limonites have been converted into mag- 

 netites in precisely the way in which the same results have been ac- 

 complished artificially ; but the manipulation of the manufactured 

 products shows that the metamorphosis is a feasible process, and by 

 no means of difficult accomplishment in Nature. 



In a review of a report by the author, in which this theory of iron- 

 ore origin is elucidated, Prof. Dana objects 1 to its value, because 

 " carbonic acid, which does now some of the work of iron-transporta- 

 tion, may have done far more then," on account of its presence in the 

 atmosphere in great abundance. No doubt exists as to the assistance 

 afforded by carbonic acid in this work, but this fact only confirms the 

 truth of our argument, since no chemist will allow that carbonic acid 

 can remove the iron-rust from the soil without the help of some deox- 

 idating agent, such as vegetation. The chemical change for which we 

 require the presence of vegetation is the same, whether carbonic acid 

 be involved or not. Indeed, an excellent authority for the form in 

 which this change is effected is the professor's own treatise on miner- 

 alogy, 2 where he says, "The iron is transported in solution as a, protox- 

 ide carbonate in cai'bonated waters, a sulphate, or as a salt of an organ- 

 ic acid." Each of these methods requires the presence of a deoxidat- 

 ing agent like vegetation ; and nothing better has yet been suggested. 

 The iron-ores produced by volcanic ejections are of very limited 

 amount, and mingled with too much dead rock to be capable of utili- 

 zation. Nor does the suggestion of the decomposition of pyrites by 

 atmospheric agents to form limonite necessitate the origin of all iron- 

 ores in that way. 



Accepting the validity of the argument, it follows that vegetation 

 must have been extremely abundant in the Laurentian and Huronian 

 ages on account of the presence in them of enormous deposits of iron- 

 ores, as on Lake Superior, in the Adirondacks, Missouri, etc. Some 

 of the beds are hundreds of feet in thickness. 



1 American Journal of Science, iii., vol. ix., p. 223. 2 Fifth edition, p. 1*73. 



