CoaU 29 



long intervals, until all of the various coal 

 strata with their intermediate formations 

 were completed. After this we must suppose 

 that the whole was submerged to a great depth 

 and for a very long period of time, because of 

 the great number and various kinds of rock 

 formations laid down by water that lie on top 

 of the coal measures. This tremendous weight, 

 as it was gradually builded up, subjected these 

 vegetable strata to an inconceivable pressure. 

 In some places this pressure was much greater 

 than in others, which undoubtedly is one of 

 the reasons why we find such differences in 

 the structure and quality of coal. There were 

 no doubt many other reasons for differences, 

 one of them being the character of the vege- 

 table growth out of which they were formed. 

 Again, in some parts of the world these coal 

 - strata may have been subjected to a consider- 

 able degree of heat, which would change the 

 structure of the formation, and in some cases 

 drive off the volatile gases. One can easily 

 imagine that heat was thus a factor in the for- 

 mation of what is known as anthracite coal, 

 so much less gaseous than the bituminous 

 kinds. The anthracite beds seem to be denser 

 and of a more homogeneous character. The 

 lines of cleavage are not as prominent, but 

 there are the same evidences of vegetable 

 origin that we find in the bituminous forma- 

 tions. 



