190 C. W. MARSH. GEOLOGICAL NOTES 



central portion, for the distance of nearly a mile, lias a dioritic 

 hanging wall, the foot-wall being graphite-schist. Behind the 

 hanging w r all are numerous veins and lenticular masses of felds- 

 pathic and hornblendic rocks, none of which appear to be 

 metalliferous, while similar veins, accompanied by graphite on the 

 foot-wall side, are for a distance of several hundred feet from the 

 lode invariably found to carry more or h-ss the ores of silver, 

 lead, zinc, and sometimes copper. These interposed masses, 

 corresponding to veins of the first-class, have in many cases, by 

 gradual changes in their composition, passed over into those of 

 the second class ; the organic matter originally present in the 

 rocks, may have been mainly instrumental, during their meta- 

 morphism, in affecting this change. 



The open spaces between the wall rocks, in which the various 

 metallic ores have been accumulated, appear to have been partially 

 due to the subsidence of the hanging wall rocks on those of the 

 foot-wall. This is shown by the steeper parts of the lode being the 

 wider, and the hanging wall resting on the flatter parts of the 

 foot-wall. Also partly to lateral movements, the foot-wall side 

 having moved south-west, or the hanging wall north-east in the 

 direction of the fissure. This is evident by comparing the 

 irregularities of the opposite walls, which have given rise to ore 

 shoots, having generally a diagonal north-east dip along the lode. 

 In addition to the earth movements already described, which 

 apparently belong to ages and conditions long since passed away, 

 there are periodical expansions and contractions of the foot-wall 

 rocks, probably due to the differences of temperature and moisture 

 of the changing seasons. This alternate upward and downward 

 motion, besides crushing and fracturing the foot- wall and smooth- 

 ing the hanging wall by planing off its inequalities, has resulted 

 in many modifications of the vein matter by repeatedly opening 

 and closing fractured portions of the vein and its foot-wall rocks. 

 The composition of the vein itself is not only constantly changing 

 by solution and re-formation, often one metallic ore replacing 

 another, but branch and small parallel veins are gradually being 

 formed by the removal of metallic salts from the main vein and 

 adjacent rocks. The value of the lode as an ore-producer depends 

 largely on the ore shoots occurring at intervals along its course. 

 Many of these have been proved to be continuous) with the 

 exception of pinches arising from local irregularities of dip and 

 strike) from the surface to depths at present worked, that is at 

 No. 6 level 360 feet, Nos. 7 and 8 levels not having been sufficiently 

 •extended yet to reach them. There are however, good reasons for 

 assuming their downward continuation, not only through these 

 levels but to any reasonable working depths. The intermediate 

 parts of the vein, connecting the ore shoots with each other, are 



