526 REPORT OF NATIONAL MUSEUM, 1890. 



This group can not, however, be separated 03^ any sharp lines from 

 that which is to follow, inasmuch as many rocks are not the product of 

 a single agency acting alone, but are rather the result of two or more 

 combined processes. This is especially the case with the limestones. 

 It is safe to assume that few of these are due wholly to accumulations 

 of calcareous organic remains, but are in part, at least, chemical pre- 

 cipitates, as is well illustrated by the oolitic varieties. 



According to their chemical nature the group is divided into (1) Ox- 

 ides, (2) Carbonates, (3) Silicates, (4) Sulphur, Sulphides, and Sul- 

 phates, (5) Phosphates, (6) Chlorides, and (7) the Hydrocarbon Com- 

 pounds. 



(1) Oxides. — Here are shown those roeks consisting essentially of 

 oxygen combined with a base, though usually other constituents are 

 preseut as impurities. 



(a) Eed hematite. Specular iron ore. Iron sesquioxide Fe 2 3 =iron 

 70 per cent., oxygen 30 per cent. This is a fibrous, scaly, or massive 

 rock of a black, brownish, or blood-red color, and which consists essen- 

 tially of iron oxide, but often carries more or less clayey and siliceous 

 matter. It occurs in extensive beds among the older formations of the 

 earth's crust and forms a valuable ore of iron. It is represented in the 

 series by but a few characteristic specimens. The visitor is referred to 

 the ore collections for a more complete series of these and of the limon- 

 ites which follow. 



(b) Limonite. Brown hematite. Iron sesquioxide plus water. (Fe 2 3 

 ■+ aq.) : An earthy or compact dark brown, black, or oeherous-yellow 

 rock, containing, when pure, about two-thirds its weight of pure iron. 

 It occurs in beds, veins, and concretionary forms, associated with rocks 

 of all ages, and forms a valuabl ore of iron. (See Fig. 1, PI. cxxii.) 

 On the bottoms of lakes, bogs, and marshes it oftens forms in exten- 

 sive deposits, where it is known as bog iron ore. The formation of these 

 deposits, as described by Dr. Hunt,* is as follows : Iron is widely dif- 

 fused in rocks of all ages, chiefly in the form of (1) the protoxide which 

 is readily soluble in waters impregnated with carbonic or other feeble 

 acids, or (2) the peroxide which is insoluble in the same liquids. Water 

 percolating through the soils becomes impregnated with these acids 

 from the decomposing organic matter, and then dissolves the iron pro- 

 toxide with which it comes in contact. On coming to the surface and 

 being exposed to the air as in a stagnant lake or marsh, this dissolved 

 oxide absorbs more oxygen, becoming converted into the insoluble ses- 

 quioxide and floats on the surface as an oil-like iridescent scum. Finally 

 it sinks to the bottom, where it gradually becomes aggregated as a 

 massive iron ore. This same ore may also form through the oxidation 

 of pyrite or beds of ferrous carbonate. At the Katahdiu Iron Works 

 in Piscataquis County, Maine, the pyrite as it oxides is brought to the 

 surface by water and deposited as a coating over the leaves and twigs 



* Chemical aud Geological Essays. 



