542 REPORT OF NATIONAL MUSEUM, 1890. 



Plants when decomposing upon the surface of the ground give off 

 their carbon to the atmosphere in the shape of carbonic acid gas, leav- 

 ing only the strictly inorganic or mineral matter behind. When, how- 

 ever, protected from the oxidizing influence of the air by water, or other 

 plant growth, decomposition is greatly retarded, a large portion of the 

 carbonaceous and volatile matters are retained, and by this means, 

 together with pressure from the overlying mass, the material becomes 

 slowly converted into coal. According to the amount of change that 

 has taken place in the carbonaceous matter, the amount of gaseous 

 matter still contained by it, its hardness and burning qualities, several 

 varieties are recognized. 



Peat is the matter in its least changed condition as it is found in 

 bogs, and in which the plant remains are still plainly visible (speci- 

 mens 70097 from New Hampshire, 36137 from Bavaria, and 59320 from 

 Bering Island). Lignite is a form in which the woody structure is 

 less apparent though often still recognizable as in specimen 6962 from 

 France, and less distinctly in No. 35743 from the District of Columbia. 

 Bituminous coal is a soft coal containing from 25 to 35 per cent, of vola- 

 tile matter, and burning readily with a yellowish flame (specimens 

 36047, 36826, and 38224). Cannel coal is a variety of bituminous coal 

 of a fine texture and almost no luster (specimen 59376). Anthracite 

 is the hard, compact, highly lustrous metamorphic variety containing 

 only traces of gaseous hydrocarbons, but with from 85 to 95 per cent. 

 of carbon (specimen 36046). Graphite is pure crystallized carbon 

 found in the older rocks and supposed by many to be of organic origin 

 (specimen 36750). 



4. Phosphatic group. — Phosphatic Sandstone; Bone breccia; Guano; 

 Coprolite nodules. This is a group of rocks limited in extent, but 

 nevertheless of considerable economic importance, owing to the high 

 values of certain varieties for fertilizing purposes. 



Guano consists mainly of the excrements of sea fowls, and is to be 

 found in beds of any importance only in rainless regions like those of 

 the western coast of South America and southern Africa (specimen 

 69281). The most noted deposits are on small islands off the coast of 

 Peru. Immense flocks of sea fowls have, in the course of centuries, 

 covered the ground with au accumulation of their droppings to a depth 

 of sometimes 30 to 80 feet, or even more. 



Au analysis of American guano gave combustible organic matter 

 and acids 11.3 per cent. ; ammonia (carbonate, etc.) 31.7 per cent. ; fixed 

 alkaline salts, sulphates, phosphates, chlorides, etc., 8.1 per cent.; 

 phosphates of lime and magnesia, 22.5 per cent. ; oxalate of lime , 2.6 per 

 cent. ; sand and earthy matter, 1.6 per cent. ; water, 22.2 per cent. 

 (Geikie). 



Coprolite nodules are likewise the excrements of vertebrate animals; 

 those among the carboniferous shales of the basin of the Firth of Forth 

 are regarded as accumulated excretions of ganoid fishes (specimen 

 36045). 



