22 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



those who may regard the work as defective and the conclusions as 

 unsound. The study has been made solely' to find solutions of 

 problems which had perplexed the writer during more than 45 

 years. The results are presented, not because they are final, but 

 in the belief that those students who take up the investigation anew 

 at some future time, when knowledge shall have been increased, 

 will find their labor lessened by this opening of by-paths in the litera- 

 ture ; and equally in the hope that credit may be restored to some of 

 the earlier students, whose work has been forgotten or ignored. 



The autochthonous origin of coal is taken for granted in this 

 work ; argument in favor of that doctrine has been presented in the 

 writer's " Formation of Coal Beds." 



The various terms applied to fossil fuels have, in a general way, 

 sufficiently definite significance. When one hears the words peat, 

 brown coal, coal, anthracite, he recognizes each as referring to a 

 substance with which he is familiar. Museums contain specimens 

 from many localities, properly labeled, so that the names become for 

 students thoroughly definitive. Tables of comparative analyses are 

 given in textbooks, which mark oft' the limits of the several sub- 

 stances with ample distinctness. It is true that in most textbooks 

 and in most lecture courses there is proper though somewhat inci- 

 dental statement that the specimens represent, for the most part, 

 what may be termed typical forms, and that from each type in 

 each direction to the next the transition is practically imperceptible. 

 Yet that that conception lacks concreteness, the more so because 

 each appears to be characteristic of a certain stage in the earth's 

 history. But the names are those of groups, each comprised of 

 members differing greatly in chemical and physical features ; and 

 there are strange overlappings, for in the groups less advanced 

 chemically, one finds substances very similar to some in the more 

 advanced, while in the latter he occasionally meets with forms almost 

 indistinguishable from some of the former. 



Since the extent of chemical change, as a rule, increases with 

 the age of the deposit, it is most convenient to consider the fuels 

 in the order of their occurrence in time. 



