A PLEISTOCENE MANGANESE DEPOSIT. 28 1 



the calcareous material with which the Golconda manganese 

 deposit is interbedded represents the lithoid tufa of Russell, and 

 that the manganese itself is a local deposit not necessarily char- 

 acteristic of the variety of tufa with which it is associated. In other 

 words, the deposit represents a lenticular bed of manganese ore 

 interstratified with a calcareous sediment, the latter having been 

 chemically deposited from supersaturated lake waters. It will 

 be seen in Fig. 2 that the manganese deposit occupies a basin in 

 this tufa, that the basin was originally cut off on the east side by 

 the rocks that formed the old shore line, and that it was bounded 

 on its west side by the outer edge of the tufa terrace. Between 

 these limits it extended a short distance up and down the lake 

 shore. This position, as well as the nature of the ore, both tend 

 to show that the bed was originally laid down as a shallow water 

 deposit and subsequently covered over by a tufa similar to that 

 which underlies it. 



It seems possible that the origin of the ore deposit was a local 

 accumulation of manganese precipitated from spring waters. In 

 support of this supposition it may be stated that at the town of 

 Golconda there are, at the present time, a series of hot springs 

 depositing a sinter highly charged with oxide of manganese. 

 The source of this manganese in the spring waters may have been 

 in the igneous rocks which cover large areas in the region in 

 question, and give strong reactions for manganese. Another 

 possible source of supply may have been in the stratified rocks 

 already described as forming the mass of the mountain on the 

 slope of which the deposit is situated, as both the quartzite and 

 the limestone contain small quantities of manganese. The igne- 

 ous rocks, however, contain a larger percentage of this material 

 than the other rocks. 



As regards the mode of precipitation of the manganese, it is not 

 probable that the ore was deposited simply by the gradual desic- 

 cation of the lake waters, as was the case with the lithoid tufa 

 enclosing it, since, if this had been so, a far more general distri- 

 bution of manganese than is seen in the tufa of the Lahontan 

 basin would be expected. It seems more probable that the 



