a 
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rie 
et 
Parr II. § vi] DETERMINATION OF ROCKS. 175 
found to have gathered again to a thickness of several inches. 
According to Ehrenberg, the formation of bog-ore is due, not merely 
to the chemical actions arising from the decay of organic matter, 
but to a power possessed by diatoms of separating iron from water 
and depositing it as hydrous peroxide within their siliceous 
framework. 
Aluminous Yellow Iron Ore is closely related to the foregoing. 
It is a mixture of yellow or pale brown hydrated peroxide of iron, 
with elay and sand, sometimes with silicate of iron, hydrated oxide 
of manganese, and earbonate of lime, and occurs in dull, usually 
pulverulent grains and nodules. Oceasionally these nodules may - 
be observed to consist of a shell of harder material, within which the 
yellow oxide becomes progressively softer towards the centre, which 
is sometimes quite empty. Such coneretions are known as etites or 
eagle-stones. ‘This ore oceurs in the Coal-measures of Saxony and 
Silesia, also in the Harz, Baden, Bavaria, &c., and among the 
Jurassic roeks in Eneland. 
Clay-Ironstone (Spherosiderite) has been already (pp. 83, 116) 
referred to. It occurs abundantly in nodules and heds in the Car- 
boniferous system in most parts of Europe. The 
nodules are generally oval and flattened in 
form, varying in size from a small bean up to 
concretions a foot or more in diameter. Inmany 
cases they contain in the centre some organie 
substance, such as a coprolite, fern, cone, shell, 
or fish, that has served as a surface round which 
the iron in the water and the surrounding mud F ROR. ae came 
Le . QDULE OF CLay- 
could be precipitated. Seams of clay-ironstone  jponsron. 
vary in thickness from mere’ paper-like partings 3 
up to beds several feet deep. The Cleveland seam in the Middle 
Lias of Yorkshire is about 20 feet thick. Inthe Carboniferous system 
of Scotland certain seams known as Blackband eontain from 10 to 52 
per cent. of .coaly matter, and admit of being calcined with the 
addition of little or no fuel. They are sometimes crowded with 
organic remains, especially lamellibranchs (anthracosia, anthracomya, 
&ec.) and fishes (rhezodus, megalichthys, &c.). 
A microscopic examination of some black-band ironstones reveals 
avery perfect oolitic structure, showing that the iron has been pre- 
eipitated in water having such a gentle movement as to keep the 
granules quietly moving while their successive concentric layers of 
carbonate were being deposited. Mr. Sorby has observed in the 
Cleveland ironstones an abnormal form of colitic structure, and 
remarks that one specimen bore evidence that the iron, mostly in the 
form of small crystals of the carbonate, had been introduced subsequent 
to the formation of the rock, as it had replaced some of the aragonite 

of the enclosed shells.4 
1 Address to Geol. Soc. February, 1879. 
