100 CLASSIFICATION OP THE PUBLIC LANDS. 



The first step of the geologist who has been sent into a field or 

 area that is known or has been reported to contain workable coals is 

 usually to determine what groups of rocks occur in that region and 

 then, knowing the groups of rocks that contain workable coal beds in 

 other places, to concentrate his attention on those groups, hunting for 

 coals in them. Having found the coals his next work is to determine 

 the number of groups of coals, as the same territory may contain 

 two or three or even more coal-bearing formations, though usually 

 they occur in different parts of the same field. His next step is to 

 examine the several coal-bearing formations in order to determine, 

 if possible, the nmnber of coal beds in each. This work naturally 

 leads' to the determination of the areal extent and thickness of the 

 beds. 



EXTEST ANJ> THICKNESS OF COAI. EKDS. 



Coal is a rock that has been formed from vegetal matter. The 

 vegetation may have grown where the coal bed is now found, much 

 as it grows in the great peat bogs of Europe or some of the large 

 swamp areas of this country, or it may have been washed or drifted 

 from the place where it grew to the place where it now occurs as coal. 

 In changing from a mass of decaying vegetal matter to a bed of 

 coal its volume has decreased and its weight per cubic foot has greatly 

 increased. It has also lost many of the elements of the* plants, es- 

 pecially the moisture they contained, and if during its formation 

 the coal bed was inundated by muddy waters its vegetal matter 

 may have been overlain or intermingled with sand or mud. This 

 material may have been added in small quantities at frequent inter- 

 vals, simply rendering the coal " dirty," or it may have come in at 

 long intervals and then in large quantity so as to form a blanket 

 of mud or sand, which, being covered by renewed accumulations of 

 vegetal matter, becomes a parting of clay or sandstone in the coal 

 bed. In a few places a single coal bed has been deposited in a 

 formation, and thousands of feet of other rocks have been laid down 

 beneath and above it, no other coals having been formed. More fre- 

 quently, however, where coal-forming conditions have existed they 

 seem to have recurred in such a way that a succession or series of 

 coal beds are laid down in the same area. In one region most of the 

 coals in a group deposited in that way may be thicker in the same 

 general area, and all the beds may tend to thin away from that area. 

 In another region one bed of a group may be thicker in one locality 

 and a higher bed of the same group may be thicker in another locality. 



Coal beds vary greatly in extent, ranging from pothole fillings 

 having about the shape though not the size of a kettle to flat- 

 lying beds thousands of square miles in extent. In Missouri a num- 

 ber of coal beds have a thickness in places of 90 feet or more and 



