Le transazioni . in mare è chiaramente indi-cato da coralli e altri animali marini fossili trovati inthem. Il solo apparentemente difendibile teoria come alla sorgente del minerale è che era derivato dal paese del periodo dur-ing che i letti erano depositati. Ho concepire lo zinco andlead come portato giù in soluzione dalla terra verso il mare, andprecipitated, in forma di solfuri, nei sedimenti cover-ing il fondale marino, attraverso l'agenzia dell'organico la matterof chert e dolomite letti, che arredate lo zolfo.che tutti i letti sedimentari non contengono tali zinco andlead--o

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Transactions . in the sea is clearly indi-cated by the corals and other marine animal fossils found inthem. The only apparently tenable theory as to the source ofthe ore is that it was derived from the land of the period dur-ing which the beds were deposited. I conceive of the zinc andlead as carried down in solution from the land into the sea, andprecipitated, in the form of sulphides, in the sediments cover-ing the sea-bottom, through the agency of the organic matterof the chert and dolomite beds, which furnished the sulphur.That all the sedimentary beds do not contain these zinc- andlead-ores is probably due to the fact that the purely mechanicalsediments, such as sands and clays, did not contain the organicmatter necessary to remove these minerals from solution. It is not supposed, however, that the sphalerite crystals wereoriginally as large as those which we now find. They wereprobably even microscopic at first, but some became enlarged at MM /|, - |. LBAD-DBPOSITfi 01 KORTH ARKANSAS. •- i. ;> THE ZINC-AND LEAD-DEPOSITS OF NORTH ARKANSAS. the expense of their neighbors; the latter passing into solutionand recrystallizing upon the Larger ones, in the manner pointedou1 recently by Ostwald. I have been disposed to think thatthese crystals were formed before the cherts were consolidated, and il still seems to me probable that such was the order ofevents in some instances.!- I am aware, however, thai some ofthe rocks have been altered in places to granular quartzites, probably from cherts. At least the rocks in some instances are Fig. 2. South a Arkansas Valley^, * Boston Nfs. Ozark riateau North ^_^_ — — -^zs^ —-^-^-^_—_^ —— A Generalized N.-S. Section Across the Ozark Plateau and the Boston Mountains. made up partly of quartz crystals arranged along parallelbands, and among these crystals are scattered here and therecrystals of sphalerite. In such a case it seems as though thematerials of the original rock had been altered in place withoutoblitera

Parole chiave associate all'immagine

• bo
• bookcentury1800
• bookdecade1870
• booksubjectmineralindustries