136 STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 



tions, one observes that a large proportion of the seams are " dirty," 

 and in reading descriptive notes of seams from w^hich samples were 

 taken, he finds that only in rare instances is a seam, upwards of 

 3 feet thick, clean throughout, while of thicker seams, a half or more 

 must be rejected in sampling. Even in thinner seams, selection of 

 samples requires no little skill. The testimony of observers, cited 

 in preceding pages, proves that a very great part of the Cretaceous 

 coal was formed amid conditions unfavorable to accumulation of 

 clean coal. Generally speaking, foreign materials are in partings, 

 but occasionally the mineral matter is distributed throughout so that 

 it cannot be removed by washing. 



SUMMARY. 



The facts recorded in preceding pages may be grouped to make 

 clear their bearing upon the matters at issue. 



I. TJie Distribution of Coal. — One who reads reports covering 

 an extensive area is liable to believe that caprice has determined 

 the distribution of coal. The presence of coal at one locality gives 

 no assurance that it will be found at the same horizon in others, for 

 great barren spaces exist between productive areas, so that indi- 

 vidual seams appear to have small areal extent; apparently, the total 

 area 'on which coal was accumulating at any time was a compara- 

 tively insignificant part of the whole. There is, however, an evi- 

 dent relation between occurrence of coal seams and the prevailing 

 character of the sediments, which would justify the assertion that 

 in one locality coal may be present, and that in another it is almost 

 certain to be absent. The descriptions seem to prove that coal 

 seams accumulate only under conditions such as mark great river 

 ■or coastal plains, where intervals of relatively rapid subsidence were 

 •followed by others, during which subsidence was slow ; finer ma- 

 terials were deposited upon the coarser and coal accumulation be- 

 gan. But where the deposits are fine, such as those laid down at 

 a notable distance from the source of materials and under a prac- 

 tically constant cover of water, coal is not present. 



The relations are sufficiently clear in the Upper Cretaceous of 

 Europe. Coal is of rare occurrence in England, France and west- 

 ern Germany, where the deposits, almost without exception, are 



