METALLIFEEOUS MINERAL LANDS. 149 



gist himself. In fact, such concrete occurrences of mineral are likely 

 to have even more weight in a contest than strictly geologic evidence. 

 It may pertinently be asked, then, what special qualification for 

 gathering the evidence required is possessed by a geologist compared 

 with a prospector or any other intelligent observer. 



The geologist would perhaps have no special advantage if unlimited 

 time were allowed for the examination. In f actj however, the time is 

 limited, it may be all too strictly. It is impossible within the time 

 allowed to explore every square foot of the area, and therefore obser- 

 vation must be concentrated where deposits are most likely to be 

 found. Now, in determining what places are best worth examination 

 the geologist finds abundant practical use for his knowledge both of 

 general and of local geology. His general laiowledge teaches him 

 that some geologic conditions are more favorable than others to the 

 formation of valuable deposits, and he distributes his attention accord- 

 ingly. He does not, for example, waste his time in looking for quartz 

 veins in undisturbed rocks, knowing as he does that deformation and 

 Assuring are necessary to the formation of veins. The special knowl- 

 edge of the region which he may already have or which he is prepared 

 by training to assimilate rapidly will enable him to recognize in the 

 geologic environment of the ore deposits that have been found the 

 details that are significant. If, for example, most of the known 

 deposits of a district are the result of interaction between a limestone 

 and an igneous intrusive, the geologist will presumably be quicker 

 than a layman to recognize this fact and will explore with especial 

 care the contacts between these rocks in areas near those which have 

 proved productive. The geologist, in shorty is better prepared than 

 the layman to follow clues. 



The shorter the time allowed for the examination the more decided 

 the advantage of the geologist over the man without geologic training. 

 An exploration sufficiently thorough to discover even a large propor- 

 tion of the valuable deposits in the region to be clas^fied is rarely 

 possible, and the value of many of the deposits found must be doubtful 

 before they are thoroughly explored. It is correspondingly impor- 

 tant, then that the fullest and most reliable inferences be drawn from 

 the data obtained in the field. 



The following paragraphs are intended to show what kind of 

 geologic evidence the examiner seeks. 



Country rock. — The broadest generalizations that can be made 

 regarding the influence of country rock, as indeed regarding other 

 geologic factors, are of negative character. It is safe to say that very 

 young unaltered volcanic rocks or imperfectly consolidated sediments 

 are unlikely to contain metalliferous lodes. Further than this, how- 

 ever, the probable richness of a formation bears no direct relation to 



