132 CLASSIFICATION OF THE PUBLIC LANDS. 



tions outlined in article D are comparatively simple, but those 

 made under article A are much more complex and require consider- 

 able computation. In order to reduce to a minimum the computa- 

 tions involved in determining the maximum depth a given phos- 

 phate bed of known thickness and content of tricalcium phosphate 

 may attain for the land to be classified as phosphate land, the ac- 

 companying diagram (fig. 7) has been constructed, by which each 

 individual problem falling under article A can be readily solved. 



All beds 6 feet or more in thickness are computed as 6-foot beds, 

 and all beds having a content of tricalcium phosphate greater than 

 70 per cent are computed as 70 per cent beds. In order to determine 

 from the diagram to what depth any phosphate bed whose thickness 

 and content of tricalcium phosphate are known should be considered 

 workable, the vertical line in the diagram that represents the thick- 

 ness of the bed is selected and followed to its intersection with the 

 diagonal line representing the percentage of tricalcium phosphate the 

 bed contains. From this point of intersection the actual or constructed 

 diagonal line is followed to the top of the diagram, where the maxi- 

 mum depth limit is given. Every 40-acre tract between the outcrop 

 of the phosphate bed and the line where the bed reaches this depth 

 limit should be classified as phosphate land. 



Where the phosphate rock occurs in several thin beds or groups of 

 beds sufficiently close together to be mined as a unit but with dif- 

 ferent percentages of phosphoric acid, all the material that can be 

 mined as one bed should be taken, or in case part of it consists of 

 barren rock that portion of the group that can be mined as a unit 

 and will give the greatest amount of phosphate rock should be con- 

 sidered. Where more than one bed exists that can be mined sepa- 

 rately without destroying the value of the other beds the land should 

 be classified on the basis of each bed separately, all the lands being 

 classified as phosphate land that can be so classified with reference to 

 any one of the beds. This method of calculation gives the maximum 

 amount of phosphate that can be obtained from the bed or group of 

 beds and is the method used in determining what lands should be 

 classified as phosphate land. 



As a result of action of this type, based on field examinations made 

 by the geologists of the geologic branch, withdrawals and restora- 

 tions have been recommended from time to time. 



Examples of withdrawal and restoration orders are given below : 



Depabtment of the Interior, 



United States Geological Survey, 



Washington, January. 25, 191S. 

 The honorable the Secretary of the Interior. 



Sib: Investigations by the Geological Survey in the Florida phosphate region 

 indicate that the" lands listed below lie within the phosphate area and prob- 



