STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 195 



for diatoms are of common occurrence, occasionally following im- 

 portant deposits ; a greater proportion is derived from the mineral 

 content of the coal-producing plants themselves ; but when the quan- 

 tity is considerable, the most of it is of extraneous origin. Pockets of 

 sand and silt have been reported by almost all observers. The silt 

 is often distributed intimately throughout the coal or it may be col- 

 lected in thin laminse, such as render the coal worthless, or it may be 

 in comparatively thick partings ; even pebbles of rock have been 

 reported from a few localities. 



Brown coal deposits, with rare exceptions, are broken by part- 

 ings. These may be so thin as hardly to be seen by the unaided eye, 

 yet they are distinct, for the coal separates on their planes ; they 

 may be a foot or more in thickness and composed of any material 

 transported or formed in place. Each definite parting is roof to the 

 underlying, and floor to the overlying coal. Roots, leaves and fresh- 

 water, as well as land shells have been reported from clay partings 

 in Hungary and Bosnia, leaves from New Zealand and Borneo and 

 freshwater shells from shale and marly limestone partings in France. 

 References to other regions are in preceding pages. Each parting is 

 evidence of interrupted coal-accumulation; but its thickness at any 

 place is no evidence respecting the duration of the period of interrup- 

 tion, for the thickness is a variable quantity. Russwurm notes a part- 

 ing which increases from 0.5 to 4 meters within a short distance ; near 

 Gran in Hungary is one which is 1.9 meter in one mine but 17.45 

 meters in another, only a little way off; Evans has described one in 

 Washington, which thickens from 4 to 90 feet within a horizontal 

 distance of 3,200 feet, while in another direction it quickly decreases 

 to 10 inches. Such variations are due to merely local causes as the 

 deposits, for the most part, are of very limited extent. The changes 

 are comparable to those observed in the rocks intervening between 

 coal beds, often so great as to make correlation of the coals difficult. 

 The partings, which consist largely of mineral charcoal, are im- 

 portant, as they appear to be often persistent throughout a basin ; 

 yet they are rarely more than half an inch thick. These indicate a 

 positive change in conditions which made growth of vegetation im- 

 posible and led to exposure of the peaty surface to atmospheric 



