56 SILVEU-LEAD DEPOSITS OF EUREKA, NEVADA. 



mineral after pyrite have occasionally been observed. Hematite (sesqui- 

 oxide of iron) is also present in the ore, but it is not as often met with as 

 the hydrate. 



Blende (sulphide of zinc) is found to some extent in the upper portions 

 of the mines, and is of frequent occurrence in the lower workings in con- 

 nection with pyrite and galena. It is usually a dull black cryptocrystalline 

 substance, but is sometimes crystalline. In the latter form it is found in 

 the "black" chamber between the eighth and ninth levels of the Eureka 

 mines. 



Calamine (silicate of zinc) is often met with in fine characteristic crystals 

 in connection with earthy limonite. It usually occurs at the junction of ore 

 bodies with the limestone, and in many instances is pseudomorphous after 

 that rock. 



Smithsonite (carbonate of zinc) is no doubt present in the ore and is 

 the product of the decomposition of blende, but no characteristic specimens 

 have been noticed. Zincite (oxide of zinc) is probably present, but its 

 detection is difficult on account of the admixture of iron in all the ores. 



Calcite (carbonate of lime) is everywhere found in the Eureka mines. 

 It occurs transparent, but is usually of an opaque milky color, cement- 

 ing together the crushed mass of the rock and in clumps of crystals in vuggs 

 and other cavities. Calcite is of rare occurrence in the ore itself. The 

 calcite, as well as the limestone, carries more or less carbonate of magnesia, 

 but none has as yet been found which contains sufficient of that substance 

 to entitle it to the name dolomite. 



Aragonite (orthorhombic carbonate of lime) is of frequent occurrence. 

 It is particularly plentiful in the caves and smaller cavities of the limestone, 

 where it often covers the entire roof and walls. In many places it is con- 

 stantly forming from the water which is oozing from the limestone. It 

 occurs in the form of radiating groups of acicular crystals and as fibrous 

 crusts and nodules covering the debris on the floors, as well as on the sides 

 and roofs of the caves. 



Measurement of the growth of aragonite crystals. — Some observations were made iu a 

 large cave between the ninth and tenth levels of the Eureka mine in regard 

 In the growth of these crystals, which were measured as follows: A co- 



