METALLIFEEOUS MINEEAL LANDS. 147 



deposits ai-e most common where the geologic structure is complex, 

 and as mountains are regions of upheaval and disturbance, the con- 

 nection between topography and mineral wealth is real. The rugged- 

 ness of the areas in which the greater part of the work of classifying 

 metalliferous land is done precludes the use of some methods that 

 are especially adapted to work in a flat country, such as that in which 

 distance is measured by the paces of a horse. The plane table is 

 much used in open country, and stadia traverses of roads or streams 

 are sometimes made for purposes of control. 



INVESTIGATION Off PLACEK DEPOSITS. 



Geologic relations. — Although theory is not so important in rela- 

 tion to placer deposits as in relation to lodes, the physiographic his- 

 torj' of the region may attord some guidance in the search for valu- 

 able placer ground, as the following considerations will show. 



The most important geologic principle relating to placers is that 

 concentration sufficient to make a valuable deposit has usually been 

 the result of long and perhaps repeated working over of the gravels 

 by streams. Therefore, other things being equal, well-washed de- 

 posits consisting in part of reworked older gravels would seem most 

 likely to be rich in gold. Moraines, on the other hand, or glacio- 

 fluviatile deposits close to the place of their origin are unlikely to 

 contain gold in sufficient concentration to be extracted with profit. 



With respect to the distance of the gravels from their source, how- 

 ever, a happy medium is favorable. Too great proximity is incon- 

 sistent with efficient concentration, but the gold, except in extremely 

 fine division, is not carried so far as the gravels, and too great remote- 

 ness from the source is therefore also unfavorable. Gold-bearing 

 placers are likely to show a definite relation to such areas of aurifer- 

 ous bedrock as can be outlined; they are partly within those areas 

 but extend beyond their borders as a sort of fringe. 



Testing. — Although a knowledge of the general geology and physi- 

 ography of the region may be a guide in the search for placers, the 

 classification of land as valuable placer ground is always supported 

 by more direct evidence, for the reason that the extent and value of a 

 placer deposit can be determined much more closely than those of a 

 lode. The examiner is expected to pan the gravels of all important 

 streams in the area classified in order to obtain evidence regarding 

 their possible value as placers and, incidentally, evidence regarding 

 the gold content of the bedrock from which the placers have* been 

 derived. 



Testimony of miners. — Owing to their necessary limitations, the 

 tests made by the examiner must be supplemented as fully as possible 

 by the testimony of miners and prospectors. From these the ex- 



