2\ 



ILLINOIS ACADEMY OF SCIENCE 



Figure 4. Average yearly production of Coal in decades for the 

 United States. 



to all mineral resources, including soils and waters. This is 

 perhaps logical in frontier areas of slight development. 



Most of the states have been mapped, at least in reconnais- 

 sance by local bureaus, though detailed work after modern 

 standards has been completed only for states of small area, or 

 for small fractions of the larger states. The U. S. Geological 

 Survey work has spread over much of the country as shown 

 by figure 4. 



Classification of public land by the U. S. Geological Survey 

 has become an important division of work. The purpose is 

 to determine whether certain lands sought to be acquired from 

 the Government are of the character contemplated by the 

 statute under which they are sought. Large withdrawals from 

 entry of public lands during several years, have been made by 

 the President or by the General Land Office, pending classifi- 

 cation of the land as to its mineral character. 



The Land Classification Board of the Survey, comprises 

 eleven geologists and engineers, and twenty-three others of 

 minor grade. The work is done by sections devoted to the fol- 

 lowing subjects: Coal, oil, phosphate, metallic ores, water 

 power and irrigation. The basis of classification and valua- 

 tion of coal land is logical and depends chiefly on the thickness, 

 depth, and heat value of the coal. Phosphate beds containing 

 less than 30 per cent of tricalcium phosphate are considered to 

 be non-mineral. Beds containing 70 per cent or more, and 

 measuring 6 feet or more in thickness, are reserved to a depth 

 of 5.000 feet. Intermediate grades are reserved to lesser 

 depths. 



