tical gash four or five inches wide, is filled with conglomerate, connecting with the conglomerate below, and tapering irregularly to a point on the upper surface of the exposure. It is clear, as Professor Van Hise has suggested, that the large mass of granite was a partly detached block of the irregular surface upon which the conglomerate was laid down, and that the sedimentary material at A and B has sifted into cracks existing in it at that time.

VI. SUMMARY.

I. We have near Republic a conglomerate which from its relations must lie at the base of the Lower Huronian, and cannot possibly be Upper Huronian.

2. This conglomerate rests in visible contact upon granite, and is a basal conglomerate;-i. e., it contains numerous waterworn fragments of the granite upon which it rests.

HENRY LLOYD SMYTH.

A PLEISTOCENE MANGANESE DEPOSIT NEAR

GOLCONDA, NEVADA.1

THE LOCATION OF THE DEPOSIT.

GOLCONDA is a small settlement in northern Nevada, in the valley of the Humboldt river, on the line of the Central Pacific Railroad. A deposit of manganese ore occurs about three miles northeast of the town, on a part of the Havallah Range locally known as the Edna Mountains, and a short distance south of where the Humboldt river has cut its channel through the range. The deposit is small and of no great commercial value, but it is of interest both in the nature of the ore and in its geologic relations.

THE NATURE OF THE ORE.

The ore is a massive, black, glossy oxide of manganese with a hardness varying from 3 to 4. It is generally of a more or less porous structure, often containing cavities lined with mammillary or stalactitic forms, and it sometimes shows apparent signs of bedding. In places it is soft, earthy and pulverulent and contains angular fragments of sandstone, shale and limestone from a small fraction of an inch to several inches in diameter. Sometimes it is stained brown by iron.

The following analysis by R. N. Brackett, Chemist of the Geological Survey of Arkansas, shows the composition of a specimen of this ore dried at 110°-115° Centigrade.

Analysis of Manganese Ore from near Golconda, Nevada.

'This deposit was examined by the writer while investigating the manganese resources of the United States and Canada for the Geological Survey of Arkansas, and was first described in Vol. I. of the Geological Survey of Arkansas for 1890, J. C. Branner, State Geologist, R. A. F. Penrose, Jr., Assistant Geologist.

It will be seen by the analysis that the ore is an impure oxide of manganese, being possibly a mixture of the peroxide and sesquioxide, though the impurities obscure its true nature. The most remarkable feature of the ore is the considerable amount of tungstic acid present, comprising 2.78 per cent of the ore and corresponding to 2.20 per cent of metallic tungsten. The form in which the tungsten exists in the ore is uncertain. It is possible that it may exist as a tungstate of manganese or iron, or of both, or perhaps of one of the other bases present. It may either have been deposited from solution with the manganese, or it may have been brought in as detritus from an outside source during the deposition of the ore, in the same way as the fragments of rock were brought into the deposit.

Though from a mineralogical standpoint the ore is impure, yet for commercial purposes the analysis shows a good grade of manganese ore, and the presence of the tungsten would give additional value to the ore in the manufacture of certain kinds of hard steel.

THE NATURE OF THE DEPOSIT.

The ore occurs as a lenticular deposit imbedded in a soft white or buff colored calcareous tufa which contains fragments of sandstone, shale and massive limestone similar to those found

There is more than a trace of cobalt present but the amount was not determined.

in the ore and often in sufficient quantities to form a breccia. This material composes a small knoll on the lower slope of the mountain, and lies on the upturned edges of underlying shale. The association of the manganese and the tufa is shown in Figure I, while the relation of the deposit as a whole to the Edna Mountains is shown in Figure 2. The first figure represents the small knoll on the left hand side of the second figure.

W

B

FIGURE 1.-Section through the Golconda manganese deposit.

FIGURE 2.-Section showing the relation of the Golconda manganese deposit to the

rocks of the Edna Mountains.

A. Quartzite. B. Shale. C. Limestone. D. Manganese-bearing deposit. Horizontal scale: 1 inch = 500 feet. Vertical scale: I inch

these scales are only approximations.)

=

300 feet. (Both of

The outcrop of the ore bed appears as a horizontal black band along the side of the knoll facing the mountains, and is very variable in thickness, in some places being represented only as a black line in the white material enclosing it and in others

widening to a maximum, where exposed, of three and a half feet. On the west slope of the knoll the ore bed is not seen at all, the only trace of it being an occasional black stain or dendrites in the limestone along the line where it should outcrop if it extended through to this side. The bed also thins out to the north and south, the whole length of the outcrop being only about 400 feet. East of the outcrop of the ore, the knoll is cut sharply off, as shown in Figure 2, by a rocky area which separates it from the mountains. It will thus be seen that the amount of ore here is limited, and it is probable that the area underlain by it does not cover more than a few acres.

Beneath the ore bed, as seen in one of the small pits that have been made on the deposit, the calcareous material is soft and partakes of the nature of a marl, while above, it is often much harder and has in many places become coarsely crystalline. The crystillization seems to have taken place in spots in the bed, and frequently bodies of crystalline material are surrounded by, and blend into a massive and softer tufa of the same composition.

The fragments of sandstone, shale and gray limestone found in this deposit are of the same nature as the beds of those rocks which comprise the mountain to the east and are undoubtedly derived from them. The pieces of limestone are so markedly different from the calcareous bed enclosing them that they cannot be confounded with it. The rock fragments are of unequal distribution in the deposit, both laterally and vertically, sometimes composing almost half of it, and sometimes being almost entirely absent. They vary from a fraction of an inch to several inches in diameter and are indiscriminately mixed.

The age of the rocks composing the part of the Havallah Range lying east of the manganese deposit is represented as Star Peak Triassic on the map accompanying the Survey of the Fortieth Parallel.' As shown in the section given above they are 'U. S. Geol. Exploration of the Fortieth Parallel; Clarence King, Geologist in charge; Vol. I., Systematic Geology, map III., Pre-Mesozoic and Mesozoic ExpoSee also report of Arnold Hague, Vol. II., Descriptive Geology, page 680.

sures.