METALLIFEROUS MINERAL LANDS. 151 



Structure.— The geologic structure of most mining districts is 

 complex, and in regions of nearly horizontal unfaulted strata metal- 

 liferous lodes are scarce. Deformation, by giving rise to fault fis- 

 sures and openings along bedding planes, probably determines the 

 location of most lodes and is therefore favorable, in general, to 

 the deposition of metalliferous ores. 



Faulting is of greater importance in the formation of lodes than 

 folding. Some faulting is doubtless a prerequisite to the formation 

 of fissures transverse to the bedding, although fault fissures along 

 which great displacement has been effected are not commonly filled 

 with veins of commercial importance, for, inasmuch as they are 

 likely to be choked with gouge and subject to repeated movement, 

 they are less likely to be ore bearing than fissures along which the 

 displacement has been slight or even imperceptible. Veins are 

 accordingly looked for near and parallel to faults of large throw 

 rather than along those faults themselves. That sort of faulting 

 which gives rise to a rather coarse breccia is perhaps most favorable. 

 At any rate, zones of brecciation afford the best clue for the tracing 

 of small faults and are likely to be mineralized. They are therefore 

 looked for and examined with especial care. 



Folding, as well as faulting, may give rise to openings in which 

 ores may be deposited. Saddle reefs constitute the most common 

 type of deposits in openings thus formed. 



Outcrops and float of lodes. — Outcrops of lodes afford the most 

 direct evidence of mineral value that can be observed in the field 

 apart from actual development, and they are therefore located and 

 described with as great care as is practicable. If the deposits are 

 veins, the direction and degree of persistence are determined if pos- 

 sible ; this determination is particularly important if the classification 

 must be close in the matter of location, but it is also important if in a 

 neighboring region veins of a certain direction are known to be espe- 

 cially rich. Size and composition are other features noted. Size 

 affects, of course, the degree of value, but the mineral content is of 

 more immediate interest to the examiner. 



In order to recognize lodes of probable value, the examiner endeav- 

 ors to learn as much as possible concerning the appearance of the out- 

 crops of lodes containing valuable minerals and familiarizes himself 

 with the minerals likely to occur in the extremely oxidized portions 

 of such lodes. Details regarding the surface indications along veins 

 of proved importance in the region examined are sometimes obtained 

 from prospectors and miners. The importance of these indications 

 may be very great, for the minerals that make the lode valuable may 

 not appear at the surface. More commonly than not the primary 

 sulphides are represented at the surface by oxides or carbonates. 

 The thoroughly weathered portion of a rich lode may even contain no 



