18 ECONOMIC GEOLOGY 



materials. Occasionally where the gangue is quartz the layers are 

 of agate, save the center, which presents a comb-like structure of 

 interlocking quartz crystals, as shown in Fig. 17. Sometimes 

 there appears to have been successive openings and fillings of the 

 fissure both with the gangues and the metallic minerals. This is 

 considered by many geologists as conclusive proof of the filling 

 of fissures from solutions. 



Age of Veins. The age of veins is determined by the manner 

 of th'eir intersection. The intersecting vein is always younger 

 than the intersected. The geological period to which fissure 

 veins belong must be determined by the fossil content of the as- 

 sociated terranes and by the stratigraphical position or the litho- 

 logical similarity of the contiguous areas in which the fissures were 

 formed. The filling of the fissure with gangue and metallic min- 

 erals is a slow, subsequent operation. 



Classification of Ore Deposits. The classification of ore de- 

 posits is a matter of convenience. It generally depends upon the 

 purpose desired. They may be classified as to their mode of 

 occurrence, as fissure, lens-shaped, bedded, etc. The following 

 classification is based mainly upon use. Metals, precious and 

 useful. 



PRECIOUS. Gold, silver, platinum, etc. 



USEFUL. Copper, iron, aluminum, zinc, and lead. Fuels: 

 coal, petroleum, gas, naphtha, paraffine. Lubricants: graphite 

 and oil. Structural: granite, limestone, sandstone, clay. Orna- 

 mental: phosphates, onyx, marble, amber. Fertilizers: limestone, 

 marl, feldspars, phosphate. Explosives: diatomaceous earth. 

 Miscellaneous: asbestos, paint. 



They may be classified as to origin for the origin of economic 

 products is as widly different as the products themselves. Prof. 

 J. F. Kemp gives the following terse order: Solution, igneous, 

 suspension. Prof. Franz Prosepny gives them Idiogenous, that 

 is contemporaneous, xenogenous, that is later than the rock. 

 Prof. W. O. Crosby gives them Igneous, aqueo-igneous, aqueous. 



The following classification has been arranged by W. H. Weed. 



A. Igneous magmatic segregation, 

 (a) Siliceous. 



1. Masses. Aplite masses. Ehrenberg, Shartash. 



2. Dikes. Beresite or aplite. Berezovsk. 



3. Quartz veins. Alaska, Randsburg, Black Hills, S.D 



