250 THE PENOKEB lEON-liEAlliNG SERIES. 



tion was greatly assisted by means of terrestrial life. That there was marine 

 life at this time, as will be seen, we have strong evidence. Of terrestrial 

 life we have no such proof, bnt it is highly probable that life existed 

 on the land, and if so, the organic acids would be of assistance in decom- 

 posing the rocks and taking iron carbonates into solution. The iron is a 

 carbonate rather than a hydroxide, for the same reasons that the bedded 

 carbonates of C^arboniferous times are so. It is usually taken for granted 

 that in the Carboniferous deposits the presence of -ft large amount of organic 

 matter ex])lains the ])resence of the iron as a carbonate. Whether the iron 

 was originall)' precipitated as a carbonate, or was decomposed and precipi- 

 tated as a hydrated sescjuioxide, just as limonite now forms from iron car- 

 bonate in places where bog ore is depositing, is uncertain. If the latter is 

 taken to V)e the case (and it is perhaps the more probable supi)osition), it is 

 necessary to b(die^'e that the organic matter with which the limonite was 

 associated later reduced the latter to the protoxide, and b)- its decomposition 

 furnished the carbon dioxide to unite with this protoxide and thus reproduce 

 iron carbonate. Anal}-ses of the carlionates of the Penokee series show 

 conclusively that there still reniains in these rocks quite a large jiercentage 

 of organic matter. Also in the thinly bedded argillaceous slates above 

 them the percentages of hydrocarbons are at times (|uite large. Some of 

 the black slaty carbonates and black slates of the other iron-bearing 

 series in the Northwest remarkably resemble the black carbonaceous slates 

 of the Carboniferous. That carbon in the form of graphite could be pro- 

 duced in other ways than by life may be conceded ; l)ut it will hardly be 

 urged that the finely disseminated carbon and hydrocarbons in these slates 

 is other than of organic origin. 



Text-books connnonly explain chert contained in limestones as of 

 organic origin More recently it has been maintained by a number of 

 writers that this chert is a chemical sediment, which has entered the i-arb(tn- 

 ates as a pseudomorph shortly after its deposition. The evidence that an}- of 

 the chert is not of organic origin is of a negati^•e character; but, as has been 

 said, it is not at all iiiipos.silde that the crust of the earth had a higher tem- 

 perature at the time of the formation of these rocks tlian at present. If so, 

 we need not necessarily go to life to account for this silica. What a power- 



