CLASSIFICATION OP MINEEAL LANDS. 103 



large area in western Pennsylvania, becomes very irregular in south- 

 ern Ohio, so much so that it has been wrongly identified, and it is 

 only within a few years that the bed there identified as the Pitts- 

 burgh has been shown to be another coal higher in the series. Like- 

 wise, in the Illinois-Indiana field coal V, which is probably the most 

 persistent bed in that field and can be traced along its outcrOp for 

 thousands of miles with great regularity, pinches out in parts of 

 Greene County, Ind., close to other localities where it shows its 

 greatest thickness; and, again, in Warrick County it loses its usual 

 characteristic roof and is split into two distinct beds. In the same 

 way many of the coals in the West that on casual scrutiny appear to 

 be persistent prove, when studied in detail, especially in connection 

 with mining operations or where close prospecting has been done 

 with a drill, to vary considerably both in thickness and in distance 

 apart. For example, at Castlegate, Utah, four beds are at one 

 point separated by 60 feet, 20 feet, and 14 feet of strata, whereas a 

 short distance away these intervening rocks pinch out and the four 

 beds, which separately are on the average only about 3 feet thick, 

 come together to make a single bed 12 feet thick. Drilling in that 

 field has shown that all the beds tend to be very irregular, splitting 

 and combining again and changing in thickness, so that in tracing 

 them from drilling to drilling it may be found that though each core 

 shows certain workable beds, yet a bed that is thick at one point is 

 thin at the next. Detailed work in many of the eastern fields where 

 extensive mining operations have afforded minute data in regard to 

 the coal, or where thousands of dollars have been expended in 

 drilling, has shown that even the most persistent of the beds are sub- 

 ject to variations, so that a bed which can be traced from one mine 

 to another over a whole county and which- may show a variation 

 of only a few inches from mine to mine may suddenly, in mining 

 parlance, " go to pieces." It is therefore the special work of the 

 field geologist not only to locate the coal ©n the ground but to make 

 as detailed a study as possible of its thickness and extent in order 

 to learn just how far it maintains a workable thickness, how per- 

 sistently it maintains a given thickness, and whether it is likely to 

 vary greatly from point to point and also to determine, so far as he 

 can, any or aU of its features. Space does not permit the detailed de- 

 scription here of all the possible irregularities that may occur in a coal 

 bed, for which the field geologist must be on the lookout. 



In some fields the coal beds are exposed in cliffs or steep slopes in 

 such a way that the coal may be seen almost continuously for many 

 miles. In such places the geologist takes the opportunity to make 

 a careful study of the regularity or irregularity of the coal beds with 

 reference both to their thickness and to the variability or regularity 



