LINDGBKN.] VEIN STRUCTURE. 647 



resulting in alteration of the country rock and deposition of a net of 

 quartz in the interstices. Ground-up mud often fills the fissure, and 

 the result of the action of the solutions on this will be a mass of grains 

 of altered rock, cemented as in a sandstone by quartz. 



STRUCTURE OF THE VEINS. 



The existence of fissure veins is primarily due to one or more fault 

 planes, fissures or seams forming ducts for ore-bearing solutions. 

 The latter have then produced the materials now forming the vein, 

 which may be divided into (1) vein-filling, or minerals deposited in 

 the open spaces along the fissure, and (2) metasomatically altered 

 country rock. Though it is not in every case possible to strictly sep- 

 arate the two classes, as a rule it can be done. Many of the puzzling 

 questions in regard to veins and vein-filling may be solved if this dis- 

 tinction is made and carefully applied. Products of attrition, often 

 present in quartz veins, belong, as a rule, to the second class of 

 materials. The vein-filling which ordinarily constitutes the ore is 

 composed of various sulphides with a gangue of more or less quartz 

 and calcite. Naturally it occurs chiefly along the fault planes and 

 seams. At Willow Creek, for instance, the seams consist of nearly 

 solid sulphides with a little quartz and calcite. These largely rep- 

 resent filling, but are probably, to a minor extent, formed by replace- 

 ment of the country rock immediately adjoining the fault planes. 

 At other localities, as at Black Hornet and Shaw Mountain, the ore 

 consists of quartz-filling exclusively, with scattered grains and masses 

 of rich sulphides and native gold. 



The typical fissure vein may be regarded as a single break or 

 fissure along which, through faulting, more or less continuous open 

 spaces were formed and subsequently filled with ore. On both sides 

 of this filling there is a gradually fading zone of alteration of the 

 country rock. 



In many regions the typical simple fissure vein is relatively rare. 

 The country rock may be cut by one or several fault planes, along 

 which only small open spaces have formed and around which there is a 

 wide belt of altered country rock. The ore, then, mainly accumulates 

 along these planes, largely by filling, partly, also, by metasomatism 

 of the adjoining rocks. Again, there may be a shattered zone adjoin- 

 ing one or more fault planes. The rock is then traversed by a com- 

 plicated system of seams, and large areas of the country rock may be 

 altered. In this case, again, the seams generally contain the gold 

 and the whole seamed rock mass may form a large ore body of low 

 grade. 



On PI. XC (p. 650) a few types of the fissure veins occurring in this 

 region are diagrainniatically represented. 



