148 CLASSIFICATION OP THE PUBLIC LANDS. 



aminer may learn the tenor of the gravels, the probable extent of the 

 pay dirt, and whether mining has been profitable. This testimony 

 may be taken under oath, if the examiner has power to administer it, 

 and should be corroborated, so far as possible, by assay certificates 

 and other evidence. A geologist is able also to criticize the testimony 

 in the light of geologic facts and would perceive, for instance, that 

 an assertion that morainal gravels had been worked with profit would 

 be in especial need of corroboration. 



INVESTIGATION OF LODE DEPOSITS. 



Kinds of evidence available. — The evidence which must determine 

 whether land is to be classified as mineral because of lode deposits 

 may be subdivided as follows: (1) General geology; (2) occurrence 

 of valuable minerals or gangue minerals, disseminated or in veins, 

 found in outcrops or float; (3) prospects and mines; (4) assays; (5) 

 history of the region. Evidence of the first two classes is especially 

 important in regions that have not been thoroughly prospected. 



General geology. — Certain geologic conditions, such as fissuring of 

 the rocks, are generally recognized as favorable to the deposition of 

 ore bodies; others, such as lack of deformation and very young 

 country rock, are unfavorable. But ore deposits are the result of a 

 happy combination of several factors, and it may be difficult to con- 

 sider these factors separately and determine their relative weight. 

 Of two districts geologically similar, one may be rich and the other 

 poor in ore deposits, because of some difl'erence not readily perceived. 

 The most general cause of such differences, perhaps, is the variation 

 in what Chamberlin and Salisbury call the " diffuse regional concen- 

 tration " of the several metals, whether in sediments or magmas, 

 which is presumably the cause of metallogenic provinces. 



Now, it is evident that geologic conditions, including obscure con- 

 ditions that may be largely determinative, are more likely to be 

 similar in neighboring than in widely separated areas. Neighboring 

 districts are the more likely, for one thing, to be in the same metal- 

 logenic province. Therefore, in judging whether a given geologic 

 condition is favorable or not, the investigator should especially in- 

 quire whether it seems to favor the deposition of ores in the vicinity 

 of the area to be classified. For example, if ore bodies are known in 

 one locality at the contact of a batholithic intrusion with a certain 

 limestone formation, it is probable that they will be found at the 

 contact of the same rocks in neighboring localities. 



This reasoning by analogy may form one of the arguments in sup- 

 port of a mineral classification, but it can rarely be made the sole 

 ground for such classification of a large area, which can hardly be 

 successfully defended against a contest unless it is supported by some 

 definite discoveries of mineral, made by prospectors or by the geolo- 



