gjQjjjcH.] GEOLOGY TESMINOLOGY OF MNING^ 277 



bedded deposits are more fi?eqnently met -wltli, and ii; Is of them -tlie 

 report on Colorado:) miueg ■s'xM treat almost exclusively, 



An ore-vein ig a tabuloid body of mioeral matter, aggregated so as to be 

 entirely distinct from tbe surrounding rock— from tlie "^country ^—in. ita 

 bthological and chemical character as vrell as in its physical struxjtnre. 

 It is necessary that a fissure— or filled — a vein should be bordered, on 

 either side, the sides being called " walls." As the greater number of 

 veins show an incline to one side or the other, it has become customary 

 to term the wall upon which it rests the "foot- wall;" the one resting 

 upon it, the " hang-ing-wall." Frequently these walls are of a diffierent 

 lithological and geological character, as at the Winnebago mine, where 

 the foot- wall is gneiss ; the hanging- wall granite. In that cas© the vein 

 is termed a " contact- veim" 



The line indicated by an outcropping vein, if cut off horizontally, is 

 called tlje ^ course," or " strike," of a vein ; variation from the horizontal 

 is the ^' dip;" the thickness is measured from wall to walk 



In distinction between a vein and a stratum, the former must be re- 

 garded as representing the fiUing material of a fissure in the adjoining 

 rocks, whereas the latter — a coal-bed, for instance — is merely one of a 

 succession of strata. From the nature of its occurrence, it becomes evi- 

 dent that the contents of fissures must be younger than the surrounding 

 " country;" whereas, in the case of a stratum, it must be said that it is 

 younger than the underlying but older than the superincumbent strata, 

 provided the beds have not been overturned. An exception to this rule 

 takes place when an ore-deposit is formed by pseudomorphic action. 



Massive deposits may be regarded as local widening of veins, but 

 more correctly as segregations. 



Impregnations sometimes form deposits of large bodies of ore, as, for 

 instance, the tin-mines of Saxony and Bohemia, but rarely produce a suf- 

 ficient amount of metal to prove valuable. 



Superstratoid deposits owe their origin mainly to mechanical action, 

 but in part to chemical. To the former class belong deposits of gold, 

 silver, platinum, tin, &c.; to the latter, bog-iron ore, bog-manganese, &c. 



Veins are subject* 'to frequent and often serious disturbances, mostly 

 demonstrated by having caused or causing dislocations, in which case 

 they are of mechanical nature, while other disturbances occur, attributed 

 to chemical action. Dislocations often cause intersections of two veins, 

 (PI. A, Fig. 1,) of which- the disturbed one is the older; the younger, 

 disturbing, keeping its course. The disturbing vein can either be larger 

 (PI. A, Fig. 3) or smaller (PI. A, Fig. 2) than the other one. An occur- 

 rence that can very readily be mistaken for crossing is termed " dragging," 

 (PI. A, Fig. 5;) and it is important in some cases to determine whether 

 an actual crossing takes place, or merely a drag is. found. Two veins 

 may approach each other at a small angle, touch, and remain in contact 

 for some distance, and then each- one turn off again to its own sjde ; or 

 they may cross and drag on either or both sides of the crossed vein, 

 (PI. A, Fig. 6.) If the one vein^ is rich and the other one poor — at the 

 same time, however, of equal thickness and similar miueralogical char- 

 acter — it becomes necessary to determine this point. It may also occur 

 that several veins cross several others, (PI. A, Fig. 4,) in which case, 

 however, all the above-stated conditions remain unchanged. 



Veins frequently ramify, sometimes the several parts returning to the 

 main vein again, sometimes pinching out entirely. The included por- 

 tions of "country" are termed " horses," (PI. B, Fig.l.) Usually the termi- 

 nation of veins is effected in three ways : by ramification, (PL B, Fig. 2,) 

 where it splits up into a number of smaller veins that gradually thin 



