Bulletin of the United States Geological Survey--The Nature and Origin of Deposits of Phosphate and Lime . h decayed, veryphosphatic siderite. This deposit covers but a small area and con- (488) INTRODUCTION. 15 sists of a patch of limestone about fifteen feet thick, which has beenconverted into siderite by the iuleaehing of iron-bearing waters fromthe ferruginous Ohio (Devonian) shales which formerly overlaid thebed. Since the escarpment of the Ohio shales retreated beyond thisbed it has been subjected to oxidation and is now in the main convertedinto a much decayed limonite. Beneath
Bulletin of the United States Geological Survey--The Nature and Origin of Deposits of Phosphate and Lime . h decayed, veryphosphatic siderite. This deposit covers but a small area and con- (488) INTRODUCTION. 15 sists of a patch of limestone about fifteen feet thick, which has beenconverted into siderite by the iuleaehing of iron-bearing waters fromthe ferruginous Ohio (Devonian) shales which formerly overlaid thebed. Since the escarpment of the Ohio shales retreated beyond thisbed it has been subjected to oxidation and is now in the main convertedinto a much decayed limonite. Beneath this limonite there is a green-ish, argillaceous sand which contains frequent nodules of lime nodules are smooth-surfaced and not unlike some of the nodulesfrom the Carolina district. They contain as much as 92 per cent, of limephosphate. It seems likely that these nodules were formed by the leach-ing out of the lime phosphate from the overlying ferruginous layers,which has completely removed the lime carbonate, but has not removedthe whole of the less soluble lime phosphate (Fig. 1).. Fig. 1. Section at Olympia, Bath County, Kv. (Preston ore bed). A, soil; B, limonite iron ore,-C, siderite iron ore; D, phosphatic nodules. Scale: 1 inch = 12 feet. Although this deposit of nodules is not of sufficient abundance to haveany economic value, it is clear that we have in it an indication of a methodwhere, by a slight variation of the conditions, important beds of nodularphosphates might be found. In the horizons of the Oambro-Silurian section, or, as it is generallycalled, the Lower Silurian, there is much greater reason to expect theoccurrence of workable phosphates than in the beds immediately is likely that the most important of the Spanish deposits belong instrata of this period, and the Welsh deposits of this general age are ofnoteworthy extent. We know, moreover, that the commoner marineanimals of this part of the geological section were particularly ad
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