184 C. W. MARSH. GEOLOGICAL NOTES 



of concentration. These solutions would in consequence of relief 

 of pressure in passing from the rocks into fissures, deposit their 

 more insoluble mineral matters ont he walls, such deposits being 

 regulated, partly by the further decrease of pressure and temper- 

 ature reducing the solvent power of the solution during its upward 

 passage through the rocks, and partly by such solutions coming in 

 contact with rocks containing salts having a sufficiently strong 

 affinity for one or other of those in solution, by which their re- 

 moval would be more or less complete. That such deposits are not 

 dependent solely on heat and pressure is apparent, since we know 

 that the chemical affinities which determine such precipitates, lead 

 to similar results through a wide range of temperature and pressure. 

 This is very noticeable in the case of metallic deposits, which have 

 apparently been concentrated around certain points leaving other 

 parts of the same vein comparatively destitute of the metals. 



Having therefore briefly noticed a few of the reactions which 

 may have assisted in the filling of these subterranean fissures, 

 it will not be necessary to consider any further modifications 

 during their upward passage through rocks, which have been 

 since removed by decomposition and denudation. We may attempt 

 however, to describe such portions of the veins, as by an accident 

 of position, now appear at the present horizon. In doing so we 

 must constantly bear in mind the susceptibility to change of many 

 of their mineral ingredients, due to the reducing and deoxidizing 

 properties of dissolved organic matters in alkaline and earthy solu- 

 tions, during their downward percolation, thus forming zones of 

 decomposition and alteration reaching to greater or less depths, 

 both in metallic veins and rock masses, which must have at all 

 times preceded the surface disintegration. 



In more particularly describing the ore deposits it will be 

 necessary to divide them into (first), those apparently connected 

 with wide spread movements of the rocks, and appearing as fissure 

 veins having a considerable horizontal extension, and (second), 

 those appearing as local deposits or ore shoots, whose surface 

 extensions are generally limited to the rock in which they occur. 

 The Broken Hill line of lode may be taken as a type of the fissure 

 veins having a proved length of about four (4) miles, while the 

 length of the original main fissure is at present unknown. The 

 lode occurs in a formation of gray gneiss, being flanked on both 

 sides by hornblendic rock (chiefly diorite). In the gneiss occur 

 numerous lenticular and irregular masses of feldspathic and horn- 

 blendic rocks, chiefly representing veins of the first class. These 

 have in many cases undergone various alterations, both in com- 

 position and structure. When occurring near the lode, they are 

 generally found to be ore-bearing. 



