278 GEOLOGICAL SUKVEY QP THE TEEEITOEIES. 



out and disappear, (PI. B, Fig. 3',) by pinching out of the main vein, (PI. 

 B, Fig. 4j) and by being cut off, (PI B, Fig. 5,) which can occur as the 

 resulJfc of considerable disturbance, whereby an entirely different kind of 

 rock can cut off and obliterate the vein. Cuts-off occurring without the 

 intervention of any new rock belong to the series of common dislocations, 

 and in that case the continuation can mostly be found again. This 

 latter takes place very frequently in coal-mines, (PI. B, Fig. 6,) where the 

 beds have originally had an approximately horizontal position, out of 

 which they are thrown by any disturbance caused either by vertical or 

 lateral pressure. In ore- veins, this kind of dislocation is not so often 

 met with. 



Sliding of veins is not infrequently found, as, for instance, in the Greg- 

 ory extension, where the one wall has changed its position parallel to 

 the medial plane of the vein-body. If any irregularities have occurred 

 in the vein, a slide of this kind will tend to increase them, and the vein 

 will consist of a series of accumulations of mineral and gangue matter 

 along its former strike and dip. 



FORMATION OF VEIICS AKD LODES. 



The formation and contemporaneous or subsequent filling of fissures 

 has given rise.to a great deal of speculation. Although it may be sup- 

 posed that an^ irregular contraction of .the earth's crust would produce 

 at points fissures similar to those Observed, their form, in detail, and 

 the character of their arrangement with reference to each other, would 

 probably not be the same as is really found in nature. An explana- 

 tion of the origin of fissures based uj^on the recognition of volcanic or 

 plutonic activity seems most satisfactory, and will apply in the by far 

 greater number of cases. Earthquakes of the present day, due to vol- 

 canic activity, form fissures and series of fissures analogous and even 

 similar to those that are -now filled with ores. Without penetrating 

 the earth's crust to any considerable percentage of itsHhickness, a vein 

 will still be practically inexhaustible ia depth until machinery may be 

 so perfected as to overcome the increasing temperature as the descent 

 is made. With the help of the annexed cut, the formation of fissures by 

 plutonic activity will become apparent. If a considerable thickness — a 

 mile or more — is allowed for the crystalline rocks a, any disturbance on 

 the part of b would have a tendency to crack the strata or the masses of 

 a, thus giving rise to the formation of fissures ; and if h becomes eruptive, 

 contact-veins would he formed between the two formations. The 

 expression of this phenomenon, however, will, as a rule, be local. 

 Circumstances can occur under which dt may cover a very large area, 

 but the existence of fissures within a small compass has been observed 

 far more frequently. 



Another kind of fissures is found that owe their existence to disturb* 

 ances as well, but disturbances having a different effect. If any stratified 

 or stratoid rock is disturbed in such a manner as to separate the strata 

 from each other at one or more points, while they remain in connection 

 at others, fissures are formed between these strata, and their strike and 

 dip will be conformable to those of the strata. 



After the fissures have been formed, they will be filled. It is a well- 

 known fact that any excavation in the ground or in rock, such as a cel- 

 lar, tunnel, shaft, &c., serves to collect and partially retain the waters 

 percolating through the surrounding medium. This is infiltration. 

 Taking the term infiltration in its widest sense, it may be accepted as 

 an explanation for the filling of fissures. It remains to be decided, how- 



