KOCKS: THEIK CONSTITUENTS AND KINDS. 71 



LiMONiTE. — A brown, brownish black to ochre-yellow iron ore, consisting of iron in 

 the same state of oxidation as hematite, but combined with water : it is hence equivalent 

 to hematite plus water, Fe203 + l^HjO = iron sesquioxide 85-6, water 14-4 = 100. Con- 

 tains when pure 59-9 per cent of iron. Its powder is brownish yellow — a distinguishing 

 character. The earthy yellow variety is the common paint, yellow ocher. In the larger 

 deposits this ore is a secondary product ; that is, was made from the oxidation of iron- 

 bearing minerals in the rocks about the deposits. So named from the Greek for marsh, 

 because a common ore in marshes, marshes being the earth's smaller pockets, catching 

 what iron is decomposed out of the rocks of the surrounding hills and washed in by the 

 waters. The marsh ore is often contaminated with phosphates from organic deposition, 

 and therefore the iron it yields is usually fit only for castings. The larger deposits, 

 not of marsh origin, are commonly pure, or nearly so, from phosphates and sulphur ; but 

 they may contain sulphur when the ore has been made from pyrite. When free from 

 sulphur it is a very valuable ore, easily worked. Great beds occur in Salisbury, Conn., 

 Berkshire County., Mass., Amenia and elsewhere in eastern New York, in eastern Penn- 

 sylvania, and farther southwest, and in many other states. 



Manganite, Psilomelane, Pelagite. — Both hydrous and anhydrous oxides of 

 manganese exist. 3Ianganite is a hydrous sesquioxide, like limonite (under iron) ; and 

 psilomelane is a massive, impure ore of related character. The color is iron-black and the 

 powder black. Over the sea-bottom concretions of impure hydrous manganese oxide 

 occur, which have been named pelagite. An analysis gave 40 per cent of this oxide to 27 

 of iron sesquioxide, with 13 per cent of water, 14 of silica, and 40 of alumina. The 

 manganese is supposed to come from the pyroxene of volcanic ashes. 



Water. — Water is hydrogen oxide, HjO = Oxygen 88-89, hydrogen 11 -11 = 100. 

 But it is never pure, because of its solvent powers. See beyond, page 118. 



Organic Contributions to the Material of Rocks. 



The materials of most rocks are of mineral origin. The rocks have been 

 produced by fusion, or out of the gravel, sand, or clay, made through the 

 wear and decay of preexisting rocks ; and as the constituents drawn upon 

 were mineral, the rocks thus derived are of mineral origin. These are the 

 most common of rocks. 



But besides the material from a mineral source, large contributions 

 toward rock-making have come from the organic kingdoms, especially from 

 those divisions of it that produce hard, stony secretions. Shells and corals 

 are examples of these secretions. Animals secreted them for protection, sup- 

 port, or some other purpose ; but they were good material for rock-making, 

 and through the geological ages, when the death of animals has set them 

 free, they have been converted into limestones. Plants are the source of 

 eoal-beds. Their stems, leaves, tissues, have become gathered in favorable 

 places into beds, like a peat-bed, and after long burial have been converted 

 into coal. Further, some kinds of animal and vegetable life secrete silica, 

 material for siliceous accumulations. 



Organic materials may occur not only in deposits that are purely of 

 organic origin, but also mixed with material of mineral origin, that is, with 

 sand, clay, gravel, and the like, in various proportions ; and sometimes a few 

 organic relics are all the materials of an organic source that can be distin- 

 guished. The organic relics preserved in any rock are called fossils (from 



