A PLEISTOCENE MANGANESE DEPOSIT. 279 



composed of sandstones, shales and limestones dipping at steep 

 angles. The upturned edges of the rocks are well exposed from 

 the summit of the mountain to its base, where they are covered by 

 the small knoll or mound containing the manganese deposit. 



The crest of the mountain is composed of a quartzite which 

 is of a dark gray color, spotted with brown specks, of a granular 

 structure, very hard and cut by numerous- quartz veins. The 

 lower beds of quartzite on the slopes resemble this one in all 

 respects except that they show less trace of their original sandy 

 structure and are more vitreous. The larger part of the slope of 

 the mountain is composed of a more or less slaty shale. It is of 

 a gray or purple color, contains large quantities of thin flakes of 

 mica, has a wavy, undulating structure and in some places grades 

 almost into a micaceous or talcose schist. The lower beds of 

 shale are much thinner than this one, and in some places resem- 

 ble it in general appearance, while in others they are more cal- 

 careous and blend into limestone. The shale which underlies 

 the knoll containing the manganese (see figures) is of a light 

 yellow color on its surface exposure, and is made up of thin 

 friable laminae. The limestone beds shown in Figure 2 are all 

 of much the same character ; they are of a light or dark gray 

 color, sometimes with a reddish tinge, generally massive, though 

 occasionally showing a tendency to a semi -crystalline structure, 

 and are frequently cut by veins of white crystalline calcite. 



THE ORIGIN OF THE DEPOSIT. 



The Golconda manganese deposit is in the arid region lying 

 between the Rocky Mountains and the Sierra Nevada, and 

 known as the "Great Basin." Parts of this region, as is well- 

 known, were, in Pleistocene, or Quaternar)^, times covered by 

 several large inland bodies of water, of which lakes Bonneville 

 and Lahontan, described respectively by G. K. Gilbert^ and I. C. 

 Russell,^ were the largest. In subsequent times these were 



'Lake Bonneville, Monograph U. S. Geological Survey, No. i., 1890. 

 ^ Geological History of Lake Lahontan, A Quaternary Lake of Northwestern 

 Nevada, Monograph U. S. Geological Survey, No. XL, 1885. 



