CLASSIFICATION OF MINERAL LANDS. 101 



an areal extent of only a few hundred feet or less, being simply 

 fillings of deep holes in the rock, similar in shape to the "pot- 

 holes" that are so abundant around some waterfalls. In other 

 places, as in the Sharon field of Ohio, the coal bed has the shape 

 of a valley, with branches where side streams have come in, and 

 a single mine may follow one of these valleys for some distance, 

 the coal being confined to a width of perhaps a few hundred yards 

 but extending indefinitely up and down the valley, which in some 

 places winds tortuously. In still other places, as in the Block 

 coal field of Indiana, the coal occurs in a succession of shallow 

 basins, the beds having a thickness of 3 to 5 feet in each basin and 

 thinning out to a few inches between the basins. The bottoms of 

 the basins lie 20 to 30 feet below their rims. Some of these basins 

 are so small and so close together that the coal from* several of them 

 is extracted by a single mine, the entries being cut through the rock 

 from the lower level of one basin to the lower level of the next 

 basin. Other basins have a length of 3 or 3 miles in a northwest- 

 southeast direction and a width of one-half mile or more. Some of 

 the individual beds can be traced from basin to basin and clearly 

 recognized by peculiarities in the coal. In other fields the coal ap- 

 pears to have been deposited unevenly in little depressions that are 

 scattered irregularly over a large territory and lie at various levels, 

 so that it is not possible to trace a bed from- one point to another, 

 and each little basin must be considered as a unit. From these types 

 of irregular and narrowly limited coal beds every gradation may be 

 found to some of the relatively even and continuous beds of the 

 West, the extensive beds of the central interior coal fields, or the 

 Pittsburgh and other beds of the East, which have an outcrop line 

 hundreds or even thousands of miles in extent and were laid down in 

 a more or less continuous sheet, many of them -covering thousands of 

 square miles. Thus the Pittsburgh bed has a known extent of over 

 6;00O square miles and is found in isolated areas beyond the limits of 

 the main bed. Other Appalachian coal beds are of much greater 

 extent, some of them, as the Lower Kittanning, having many times 

 the areal extent of the Pittsburgh bed, though not its regularity. 



Certain coal beds in the Illinois fields have been traced without 

 question as to their identification over a large part of that State, 

 through a portion of western Kentucky, and through the entire 

 length of the Indiana coal field. It is believed that many of the coal 

 beds of the "Western States are also traceable over large areas. Most 

 of these widely extended coal beds have certain slight but definite 

 features or peculiarities or are associated with other rocks of dis- 

 tinctive character, so that it is possible to recognize them at any 

 point. The peculiarity of a bed may consist of some particular type 

 of parting or arrangement of partings which may hold for long dis- 



