190 STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 



is 31.70 and the volatile is 46.73; in II. the ash is 5.77 and the 

 volatile is 45.89. 



Summary. — It is now in place to gather the facts presented and, 

 if possible, to ascertain how far they may be related to the problem 

 in hand. 



The testimony throughout shows that Tertiary coal beds are no- 

 tably limited in extent, their area varying from a few square rods to 

 several hundreds of square miles — in some cases apparently even to 

 2,000 square miles; the extent being limited by the topography as 

 the deposits appear, for the most part, to have accumulated in shallow 

 ponds or in well-defined valleys. The lens-like form has been empha- 

 sized by almost all observers in every portion of the Tertiary. Un- 

 fortunately the details recorded for most localities are insufficient to 

 justify an attempt at working out the history of any bed, known to 

 exist within a large space. Detailed study is impossible at present in 

 the United States and Canada, where alone the great beds are known, 

 because those occur in regions with sparse population, where, for long 

 distances, one must depend on imperfect natural outcrops or on the 

 less definite lines of clinkered rock, caused by spontaneous com- 

 bustion of the coal. The perplexity is increased by variations in 

 thickness and composition of the intervening rocks, as well as by 

 similar variations in the coal beds themselves, which make correla- 

 tion extremely difficult. Several American observers decline to 

 regard the coal deposits as continuous in large areas and prefer to 

 describe " coal horizons." All agree as to the lens-like character of 

 very many beds ; even those who are unwilling to accept this for the 

 great beds, frankly present the frequent changes into shale and the 

 local disappearances of the coal as serious problems. Some ob- 

 servers have shown that the lenses often overlap, that a coal thins 

 out and may be replaced by another at a few feet higher or lower. 

 This feature, so characteristic of American localities, was recognized 

 by Credner and by Raefler in the coals of Prussian Saxony. 



The rocks intervening between beds of Tertiary coal may be 

 conglomerate, gravel, sandstone, sand, shale, clay, limestone or 

 alternations of such deposits. Ordinarily, these are of freshwater 

 origin, but, not infrequently, one finds layers with brackish- water 



