MINERALS OF THE UNITED STATES — PEHRSON 181 



exceptions are petroleum and natural gas, for which it was felt that 

 available data did not permit estimates of inferred reserves with any 

 acceptable degree of accuracy. 



UNCERTAINTY OF FUTURE DISCOVERIES 



In appraising the significance of present estimates of commercial 

 mineral reserves, consideration must be given to the possibilities of 

 discovery not contemplated in the inferred ore included in the esti- 

 mates. We may start with the assumption that the potential mineral 

 areas of the Nation have been heavily prospected and most of the sur- 

 face deposits or those whose presence is indicated by superficial 

 phenomena already have been found. This assumption is substan- 

 tiated by the decline in new discoveries. During the past 2 years, much 

 attention has been given to the failure of petroleum discoveries to keep 

 pace with the rapid rate of depletion. The rate of discovery of 

 metalliferous deposits has been declining at an alarming rate for half 

 a century. It is significant to note that no major metal-producing dis- 

 trict comparable to Butte, Bisbee, Homestake, or the Coeur d'Alene 

 has been brought into production in the United States since the active 

 development of the Picher-Miami lead-zinc deposits of the Tri-State 

 area about 1916. There have been few important copper discoveries 

 in the present century. Most of the large deposits producing today 

 were known before 1900. The largest discovery since then has been 

 the United Verde Extension mine at Jerome, Ariz. The production 

 from this property was relatively insignificant in terms of national 

 needs, the entire output in the 20-odd years the mine operated having 

 amounted to only a half-year supply at normal rates of consumption. 

 It is true that new mines have been developed, but the large ones were 

 the result of the application of improved technology to deposits whose 

 presence had been known for many years, and the small ones in the 

 aggregate contribute but a small part of the national requirements. 

 Future additions to the national reserve will depend largely on the 

 success achieved in utilizing known low-grade deposits not available 

 economically at present and the discovery of deep-seated or concealed 

 deposits, the presence of which is not easily discerned. Progress in 

 converting submarginal resources into commercial reserves can be 

 made through research to improve methods of extracting and process- 

 ing minerals and to reduce costs. Higher prices also can be an im- 

 portant contributing factor. 



Progress in the field of new discovery depends upon the success at- 

 tained in developing effective and economical methods for finding 

 deposits not recognizable from surface indications alone. Substantial 

 quantities of ore occurring under heavy cover have been found by 



