NEW YORK MINERAL LOCALITIES 7 



Secondary minerals 



Secondary minerals, developed as a result of chemical action 

 on previously formed rocks, are. to a large extent, deposited by 

 percolating water. With regard to their mode of occurrence 

 they may be classified as follows: (1) concretions; (2) deposits 

 lining the interior of cavities, vugs, caverns and grottos; (3) 

 vein formations; (4) minerals produced through pseudomor- 

 phism and paramorphism. 



Concretions. Concretionary deposits of mineral matter are 

 frequent in rocks of sedimentary origin. They are in general 

 formed by the deposition, in successive layers around some or- 

 ganic center, of mineral matter leached from the surrounding 

 rock. The calcium carbonate concretions found in clay beds are 

 excellent types of this form of mineral occurrence. Concre- 

 tionary forms of quartz, siderite, pyrite, chalcocite etc., are also 

 formed in sedimentary rocks. 



Deposits lining the interior of cavities, etc. The formation of 

 secondary minerals in cavities of various origin results from the 

 chemical action of percolating water on the rock adjacent to 

 and forming the walls of the cavity. The soluble mineral matter 

 is dissolved from the rock traversed by the descending surface 

 water to be redeposited, sometimes in an entirely different form 

 in the open spaces. The minerals thus deposited take the form* 

 of distinct crystallizations or of concentric, incrusting masses. 



COMMON SECONDARY MINERALS OCCURRING IX CAVITIES 



hematite barite apophyllite 



limonite celestite stilbite 



quartz anhydrite chabazite 



calcite gypsum heulandite 



dolomite serpentine harmotome 



siderite [ sulfur analcite 



aragonite datolite natrolite 



strontianite prehnite 



Vein formations. 1 Mineral veins may, with justice, be con- 

 sidered as constituting a division under the last named class of 



secondary mineral deposits; the distinctive character of the 



iThe formation of mineral veins has been very fully discussed byPosepny, F. 

 Genesis of Ore Deposits. Am. Inst. Min. Eng. Trans. 1893. p. 23-197. 



