Page:The Journal of geology (IA journalofgeology11893univ).pdf/297

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 characteristic 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 igneous 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 desiccation of the lake waters, as was the case with the lithoid tufa enclosing it, since, if this had been so, a far more general distribution of manganese than is seen in the tufa of the Lahontan basin would be expected. It seems more probable that the