126 CLASSIFICATION OF THE PUBLIC LANDS. 



over in the intense folding that has occurred. The phosphate 

 deposits therefore not only lie flat but stand at various inclinations, 

 and they lie at great depth except where the long-continued action 

 of the elements has worn away parts of the rock folds. These folds 

 are of two principal types — upfolds, or anticlines, and downfolds, 

 or synclines. In some places the folding has been moderate in 

 amount and the folds are long, regular wrinkles. In other places 

 the folding was so intense and the wrinkles were so sharp and 

 extensive that the beds are broken or " faulted " and come together 

 in irregular order. Faults may and in many places do sharply 

 separate phosphate from nonphosphate land, and where such faults 

 are concealed by later deposits, such as gravels, they may lead to an 

 apparently unwarranted difference in the classification of adjoin- 

 ing and seemingly similar tracts. Two 40-acre tracts in a section 

 may in reality be underlain by phosphate and the remaining tracts 

 may be barren, although the surface character of all may be exactly 

 similar, so that it is necessary that the field examinations, the nature 

 of which is more f uUy explained elsewhere, should involve more than 

 a study of specific tracts. 



The phosphatic series is composed of rocks which wear away 

 easily under the action of the elements, and its outcrop is therefore 

 inconspicuous; but the rocks immediately above and below it are 

 resistant and in many places stand out in ledges that are easily 

 traced. The field surveys on which the classification of the lands 

 is based are made with suificient detail and accuracy to determine 

 the distribution of phosphatic beds relative to 40-acre tracts or other 

 legal subdivisions, the thickness and character of cover, and the 

 quality and thickness of rock phosphate. The determination of the 

 factors last named has in places involved the actual prospecting of 

 the deposits by deep trenches, the longest of which was over 400 feet 

 long. Certain other preliminary surveys of a reconnaissance nature 

 have been made in order to afequire data for withdrawals and pre- 

 liminary modifications of the reserve boundaries. 



The estimated quantity of high-grade rock (containing 70 per cent 

 or more of tricalcium phosphate) included in the area surveyed in de- 

 tail to date is more than 3,000,000,000 long tons, yet it is possible that 

 such an estimate, based solely on information collected along the 

 outcrop of the beds, may be excessive. Below the surface the brown 

 phosphate of Tennessee rapidly becomes lean and grades into the 

 phosphatic limestone from which the phosphate is supposed to have 

 been concentrated by weathering. The phosphate deposits of the 

 western reserves may ultimately be found to show a similar change, 

 although they do not exhibit clear evidence of such concentration 



