ORIGIN OF THE FAULTS. 241 



as, for instance, the vein displayed at the top of the Redrock sandstone 

 quarry in Marion county. 



As an example of what took place during the period when coal was 

 forming, a case may be assumed somewhat similar to that just men- 

 tioned. Let there be imagined a swamp one-fourth to one-half a mile in 

 diameter and 200 feet in depth ; let the swamp be filled to within a few 

 feet of the water-level with half decayed vegetation ; let the region, after 

 this stage is reached, become one of slow subsidence, and let it be so 

 situated as to allow the introduction into the swamp of currents, which 

 sweep in sediments of different kinds ; then the conditions of the old coal 

 marshes are practically rei:)roduced. If the superincumbent sediments 

 should continue to accumulate and tlie peat-like mass be compressed to 

 one-tenth or less of the original bulk the process would be almost iden- 

 tical with that which seems to have taken place in connection with manv 

 of the coal beds of Iowa. It being possible for the compression and 

 diminution. of bulk to take place only in one direction, that is, vertical, 

 on account of the weight of the overlying beds, there results a broad 

 sheet of pressed vegetal remains, thickest centrally, and becoming 

 gradually attenuated toward the margins. During the process of com- 

 jDression the central part of the upper surface, wliich, just prior to the 

 influx of sediments when the swamp had reached its greatest develop- 

 ment and expansion, was on a level with the margins, is little by little 

 depressed or bent downward as the plant remains are more and more 

 compacted. At the end of the process the upper surface of the plant 

 mass at the center of the basin will be from 175 to 185 feet below its 

 original level. In the particular case assumed this would be a slope of 

 about 1 in 13 in all directions toward the center, or a dip of 5°, nearly. 

 Conceiving this area to be covered to a considerable depth with other 

 beds, the phenomena becomes practically identical with what is now 

 observed everywhere in connection with the coal beds. When the lower 

 and more extensive seams of the Coal Measures are constantly contract- 

 ing in bulk the series higher up and the associated strata, in responding 

 to the effects of gravity, become undulator\' or give rise to a series of 

 small slips. This fact may, after more extended investigation, come to 

 have an important economic bearing in the detection of valuable coal 

 beds at lower levels than are commonly worked or prospected. 



Summary. 



From the foregoing it would appear that — 



In the upper Mississippi valley there exist faults, the number and ex- 

 tent of which has never been fullv realized. 



