498 mckelvey. SEDIMENTARY MINERAL DEPOSITS [Ch. 27 



garding the structure and extent are required before tonnages can be 

 estimated or before further exploration can be conducted. Therefore, 

 it is the task of the geologist who compiles the facts to make the first 

 guess as to their meaning. 



Unless the geologic map condemns the property, information must 

 be acquired about the quality and thickness of the deposit. The num- 

 ber of intersections required to establish grade and thickness varies 

 considerably. Blanket deposits of the type already referred to are re- 

 markably uniform in grade and thickness over large areas, and their 

 quality in the mining unit under consideration often can be demon- 

 strated adequately by half a dozen samples. Other deposits may re- 

 quire channel sampling or drilling at intervals of a few feet or tens of 

 feet before the quality and thickness of the material can be established. 

 The number of samples required is one of the points that the geologist 

 must (and is best qualified to) determine. 



The prime concern in estimating tonnages of various grades of 

 material is the physical continuity of the beds. The usual stratigraphic 

 methods, ranging from the use of prominent marker beds to accessory 

 minerals and fossils, are employed in tracing the beds, and the degree 

 of success achieved often determines the reliability of the estimates. 



When the deposit is demonstrated to be discontinuous or variable in 

 grade and thickness, the accumulated measurements of thickness and 

 quality must be interpreted. This again is the geologist's province, 

 and, in the field of appraisal as a whole, it is the place where the 

 principles of sedimentation are needed most. A thorough understand- 

 ing of the origin of the material — its manner and environment of 

 precipitation and deposition — are required to reconstruct the shape of 

 the deposit, or to evaluate the importance of local variations in grade 

 and thickness. Unfortunately such knowledge is generally lacking, 

 and various rule-of-thumb or statistical methods must be relied on to 

 calculate the tonnage of various classes of materials. 



Even statistical methods cannot be used indiscriminately, but must 

 be applied in the light of the best information available on the geologic 

 nature of the deposit. If the variations in grade or thickness are of 

 a truly random nature, certain assumptions and methods are possible 

 that are not appropriate at all if such variations are found to have 

 a pattern. Even though such a pattern is not understood, or is ex- 

 plained by incorrect theory, it is by no means ignored in the estima- 

 tion of tonnages. 



One of the difficult questions which frequently arises in the appraisal 

 of mineral deposits is "What is the continuity of the ore at depth?" 

 Here again a theory of origin is required to supplement facts, but the 



