182 C. W. MARSH.— GEOLOGICAL NOTES 



oligoclase and hornblende, or labradorite and pyroxene. The 

 elimination of the alkali from such a solution, as above, might, 

 by the reactions of lime and magnesia upon each other, produce 

 such basic silicates as epidote, lime-alumina-garnet, etc., on the 

 one hand, and chloritic minerals on the other. 



That these veins have been filled by some such process as the 

 above is probable, considering how their mineral matter is that 

 generally common to enclosing rocks, while their irregular positions 

 in relation to each other, and their limited extent, both in length 

 and depth are distinguishing features from fissures, which have 

 been open channels for the free circulation of mineral solutions, 

 or the extrusion of eruptive rocks. The origin of these openings 

 may have been threefold, some may have resulted from con- 

 traction, consequent on the molecular rearrangement and crystal- 

 lization of the rocks in which they occur, and others to the 

 influence of eruptive rocks in their vicinity. The greater 

 number, however, are probably due to the rocks accommodating 

 themselves to new positions during the wide-spread movements 

 which must have accompanied their gradual subsidence and eleva- 

 tion, while the heat, partiy arising from these movements, and in 

 part due to depths at which such movements took place, may 

 have given to these rocks a sufficiently yielding nature to have 

 favoured the formation of numerous small openings rather than 

 extensive fissures. In this connection we may assume that in 

 many cases the opening and filling of these veins have proceeded 

 simultaneously, the banded structure characterizing many of them 

 marking periods during the downward and upward movements 

 of the rocks. 



The veins of the second class are often found cutting through 

 those of the first class or running parallel with them, besides 

 frequently filling either one side or other part of the same fissure, 

 showing the re-opening of many of these older veins during or 

 prior to the formation of the latter class. These veins have 

 evidently been formed at a much later date, probably marking 

 periods of elevation and removal of the upper rocks, during which 

 periods rain waters may have intervened, introducing organic 

 acids to the subterranean circulation, resulting in the removal 

 from the rocks of dissolved silica, lime, magnesia, alumina, iron, 

 and many of the more precious metals, which have been after- 

 wards deposited, as circumstances directed, either in the more 

 porous parts of the rocks themselves, or in fissures arising from 

 their movements. 



These veins may be described as consisting essentially of quartz, 

 into which, however, feldspathic, hornblendic, and micaceous 

 minerals occasionally enter ; the principal accessory minerals 

 being the sulphides of iron, lead, copper, zinc, antimony, and 



