180 C. W. MARSH. — GEOLOGICAL NOTES 



the cementing medium for the more unalterable minerals, such as 

 quartz, &c. ; and that, during the removal of the decomposed 

 material by winds and water from the higher to lower grounds, 

 these iron ores, together with the more highly siliceous parts 

 of the rock, have been liberated and subsequently arranged as 

 are now found. We must bear in mind, however, that such -con- 

 centrations of iron oxide, under favourable conditions may have 

 taken place immediately below the alluvial surface, and have been 

 subsequently laid bare, together with the preserved siliceous frag- 

 ments, by atmospheric agencies, removing the finer material from 

 the higher to lower horizons. 



Ore Deposits of the Barrier Ranges. 



It becomes necessary, before taking up the study of the ore 

 deposits in any district, to investigate closely, not only the 

 mineralogical and lithological characters of the rocks, but that of 

 their enclosed mineral deposits arising from the various evolutions 

 through which the rocks may have passed. The ore deposits of the 

 Barrier Ranges occur in crystalline stratiform rocks, represented 

 chiefly by gniess and mica-schist. It may be assumed that these 

 rocks were originally due to sediments and precipitates from: 

 ancient oceanic waters, and have been subsequently deeply buried 

 beneath the ruins of other formations, and lowered to positions, 

 where heat and pressure, in combination with alkaline solutions, 

 have produced chemical actions resulting in the molecular re- 

 arrangements and crystallization of their feldspathic, siliceous, 

 and argillaceous constituents. Since then they have had their 

 super-incumbent rocks stripped from them by denudation, con- 

 sequent on their elevation, and their further modifications have 

 been mainly due to local and superficial agencies, such as the 

 actions of oceanic and atmospheric waters, and the presence of 

 organic matters. It is plain that the various earth movements,, 

 by which these rocks have been affected, have been accompanied 

 by dislocations and fissures, which under changed conditions of 

 heat, pressure, and chemical solutions, have been filled by 

 different combinations of mineral matter, thus offering historical 

 records of the successive periods of disturbance. 



In describing the veins thus formed, we will take them in the 

 order of their apparent formation, and divide them into veins 

 of the first and second class respectively. Those of the first 

 class, apparently corresponding to the periods of most intense 

 metamorphism, closely resembling eruptive rocks, and although 

 seemingly having no connection with them, may be described as 

 siliceous (those in which quartz and alkaline feldspars predominate) 

 and basic (those made up essentially of the more basic feldspars 

 with hornblende). The relation of these veins to their enclosing 



