Ill 



timbers were brought down to sea from far inland forests by the 

 great rivers of that time, although it appears that the great mass 

 of the contents of the coal was lowland in plant types. As in peat 

 the processes of decay did not complete their full course and much 

 of the carbonaceous material of the ancient drifts has remained in 

 the seams to the present day. Chemical changes there have 

 been, chiefly in the proportion of hydrogen and oxygen in com- 

 bination with the carbon. 



Anthracite coal differs in many respects from ordinary so- 

 called humic coal. It burns with a clear flame, and emits little 

 smoke that corresponds to the lighter hydrocarbons. The mode 

 of its deposition and also of its transformation from the mother 

 substance to the present are both alike obscure. This at least 

 seems plain ; it is not derived by metamorphism from ordinary 

 coal. The likelihood is that they are perfectly distinct and are 

 the derivatives of two distinct types of vegetable mother sub- 

 stances. What the mother substance of anthracite was, we do 

 not know. 



Petroleum and oil shales have become of great economic 

 importance during the past half century and especially so within 

 the past 15 years as a result of the development of the internal 

 combustion engine. The chief petroliferous areas at present ex- 

 ploited are in U.S.A., Russia, and the foot-hills of the Carpathians. 

 The British Empire appears to be inadequately supplied. In 

 England the Kimmeridge Clay, in Purbeck and Norfolk, includes 

 rich oil-shales that await exploitation. The Midlothian oil-shale 

 industry, despite the keenest competition and many natural diffi- 

 culties, has maintained its prosperity for a long period already. 

 Oil-shales must be distinguished from oil deposits. Only excep- 

 tionally does an oil-shale yield oil without distillation, the hydro- 

 carbons being held in some sort of combination in the matrix of 

 shale or sand, as a substance, termed kerogen. Little more can 

 be said of this substance. As a rule the oil of the great oil fields 

 is held as such in the midst of a sandstone or limestone. Fre- 

 quently the rocks of the oil field are arched up into an anticline, 

 the gas being originally at the top above the oil, which in turn 

 rests upon brine filling the basin of the syncline, associated with 

 the anticline. 



Quite naturally a> great variety of theories is held with regard 

 to the origin of petroleum. These may be largely grouped into 

 two classes, the inorganic theory or metallic carbide theory and 

 the organic theory. According to the first idea the hydrocarbons 

 found in the earth's crust were originally formed by interaction 

 of water with metallic carbides at great depths, and later they 

 made their way upwards through the covering of sedimentary 

 rocks into the beds where they are now found. A variation of 

 this opinion holds that mud volcanoes may well have been active 

 in spreading out vast flows of mud rich in hydrocarbons. Certain 

 evidence bears immediately upon this theory. Mendelejeff observed 

 that many metallic carbides give hydrocarbons with water. 



