THE JMONMETALLIO MINERALS. 423 



COLCOTIIAK. 



FcA 74.62 



ALA 1^-^^^ 



MgO ;^-^3 



CaO 0.82 



SO3 •'^■17 



SiO., 1-1" 



('u() 0.20 



H.^() 1.30=99.04 



XIII. HYDROCARBON COMPOUNDS. 



1. Coal Series. 



Here are included a variety of more or less oxygenated hydrocar- 

 bons varying widely in physical and chemical properties, but alike in 

 originatiiig from decomposing plant growth protected from the oxidiz- 

 ino- influences of the air. According to tiie amount of change that has 

 taken place in the original plant material, the amount of volatile matter 

 still retained by it, its hardness and burning qualities, several varieties 

 are recognized. 



Origin.— The idea long prevalent but never entirely accepted to 

 the effect that the coal l)eds resulted from the accunmlation in situ of 

 organic matter growing on gradually subsiding marshes has of late 

 given way quite largely to another more in accord with the facts as 

 now known. 



While we have indu])itable proof that peat may and does thus origi- 

 nate, as is to be seen in many a modern peat bog, and while, too, there 

 is no doul)t as to the possibility of such, under proper conditions, 

 becoming converted into coal, still there are many facts which tend to 

 show that perhaps the most and the largest of the coal deposits are 

 due to the accumulation of transported plant remains laid down at the 

 mouths of rivers as in deltas and lagoons. They are in fact as true 

 sedimentary deposits as the shales and sandstones with which they are 

 associated. This view best accounts for the constant interlamination 

 of the coal with clay and sand, with the marked sti-atitication of the 

 coal itself, as well as the amorphous nature of the material, since, as is 

 well known, calcium sulphate, a constituent of sea water, tends to 

 decompose organic matter, reducing it to a pulplike, and at times 

 almost mucilaginous condition. 



The idea, too, long prevalent, that anthracite is but a bituminous 

 coal from which a large portion of the volatile matter has been driven 

 off l)y the heat and pressure incidental to mountain making or the 

 intrusion of igneous rocks is also in part l)eing set aside. Undoubtedly 

 anthracite may l)e thus produced and in some cases has been thus pro- 

 duced, as in the Cerrillos coal field of New Mexico, where a bitumi- 

 nous coal containing some 30 per cent of volatile matter has been locally 



