494 mckelvey. SEDIMENTARY MINERAL DEPOSITS [Ch. 27 



and receiving geologic guidance. Many state highway departments, 

 the Corps of Engineers, the Bureau of Reclamation, railroad companies, 

 smelter companies, and similar organizations are employing geologists 

 to aid in the location of suitable materials. Some of the methods used 

 in this search are discussed in other chapters in this volume and need 

 not be recounted here. Suffice it to say that the results of application 

 of geology to the search for many non-metallics has often led to a con- 

 siderable saving in the cost of materials, not only through the dis- 

 covery of materials that meet the required specifications more satis- 

 factorily than those previously used, but also especially in locating 

 deposits close to the project. 



THE SEARCH FOR EXTENSIONS OF KNOWN DEPOSITS 



Most exploration for minerals now consists in seeking extensions of 

 known deposits or seeking new deposits in known districts. There 

 are three good reasons for this: (1) geological theory is not sufficiently 

 advanced to support a search for a mineral in districts not already 

 known to contain it; (2) the presence of known mineral deposits sug- 

 gests the proximity of others as yet undiscovered (look for bears where 

 bears are known to be!) ; and (3) it is much more profitable to mine 

 a material in the vicinity of a going operation (where permanent mine 

 and mill installations have already been amortized, and where trans- 

 portation and power facilities are available) than to begin mining in 

 a new district. 



As the mode of occurrence of many sedimentary deposits is simple, 

 the search for extensions of known deposits can be guided by elemen- 

 tary geological theory. Many of the deposits (such as some coal, 

 potash, phosphate, limestone, and iron formation) were deposited as 

 blankets over areas of several hundred to several hundred thousand 

 square miles. As these materials occur at more or less definite horizons, 

 they can be traced without difficulty by the common stratigraphic 

 methods. In regions of intense folding or faulting, geologic mapping 

 is, of course, required to determine their location. Often many com- 

 plex structural and stratigraphic problems must be solved in the prepa- 

 ration of such a map, but no geological theory is required to find the 

 position of the mineral deposit in question once the map is available. 

 Positive proof is required, of course, to show that the formation con- 

 tains the mineral deposit; such proof may require drilling, trenching, 

 or other types of physical exploration. 



The irregular or lenticular nature of some sedimentary deposits 

 makes the search for them much more difficult. Many of these deposits 



