240 C. R. KEYES — CRUSTAL ADJUSTMENT IN MISSISSIPPI VALLEY. 



and form a compact sandstone wall. One of these from the Thistle mine 

 in Appanoose county is shown in figure 6. 



Origin of the Faults. — While many of the displacements in the region 

 under consideration are undoubtedly due to the ordinary causes produc- 

 ing normal faulting, there are some cases in the Coal Measures, and 

 perhaps these form the majority of the small slips, which owe their origin 

 to the gradual compression of the coal beds themselves after the original 

 vegetable accumulations had been covered by sediments. The process 

 of compression is probably in progress at the present time, though slowly 

 and at a greatly diminished rate compared with the former periods. 



When a vegetal mass is subjected to conditions favorable to the forma- 

 tion of mineral fuel in the way that most of the coal beds of the upper 

 Mississippi valley were apparently formed, there is a constant addition 

 of sediments above, increasing the superincumbent weight until it fre- 

 quently becomes something enormous ; the temperature of the mass at 

 the same time gradually riseis. The heat and pressure, aided at times 

 by other agencies perhaps, may continue to be operative for long periods, 

 the plant accumulation in the meanwhile going successively through all 

 stages of bituminization to the hardest anthracite or even to graphite. 

 In the process, through the loss of water and various gases and through 

 certain chemical reactions of the various component elements or com- 

 pounds among themselves, the bulk of the mass is very greatly reduced, 

 the amount of reduction of course depending upon the nature of the 

 swamp materials, the degree of bituminization and method by which 

 the loss of carbon is effected. According to the estimates of Maclaren, 

 in one of the Scottish coal-fields, it would take nearly 2,000 acres of 

 forest to produce an acre of coal three feet in thickness. In case of the 

 average Iowa coal bed, it has probably taken upward of thirty feet of 

 closely compacted material of the original woody growth to produce a 

 seam of coal having the thickness of four and one-half feet — the mean 

 measurement of the veins mined in the state. Ordinary anthracite prob- 

 ably shrinks to less than one-tenth of its original bulk in the course of 

 its formation, so that a bed 25 feet thick may represent between 250 and 

 300 feet of the original mass. 



In the diminution of bulk in a great lenticular deposit of vegetation, 

 the change in dimensions is chiefly in a vertical direction. Providing 

 the surface of the marsh were originally nearly horizontal, as probably was 

 usually the case, the margins would remain stationary, while the center 

 of the mass, where the contraction is naturally greatest, would be de- 

 pressed below the level of the borders, producing, when fully compressed, 

 a shallow, saucer-shaped sheet of coal. This fact is also well exhibited 

 in the coal deposits of Iowa, especially in the case of the smaller basins ; 



