The composition of the yellow-green mineral thus determined corresponds nearly with chloropal.

Diopside. In the Palmetto Mountains in Esmeralda County, Nevada, there are a few small areas of serpentine at or near the contact of granite and other rocks, either acid metamorphic lavas or cherts of Ordovician age. Associated with the serpentine are bunches of limestone, and at one point a streak of garnet and magnetite.

In the serpentine was found a considerable amount of a monoclinic pyroxene, which was separated and analyzed.

Partial analysis of pyroxene No. 323 from serpentine mass.

This pyroxene is evidently near diopside in composition, and it is possible that it is of metamorphic origin from dolomite, as has been shown to be the case at Montville, N. J., by G. P. Merrill. The serpentine is presumed to be derived from the

pyroxene.

Vesuvianite.-It is well known that at the contact of granite and limestone various silicate minerals often form from the action of heat and vapors of the intrusive granite on the calcareous material. At such a contact in the NE. part of the Silver Peak quadrangle a silicate mineral was found associated with garnet and quartz. This silicate, freed from the associated minerals by means of the Thoulet solution, was analyzed with the following results (p. 346). Boron was looked for but not detected by blowpipe test.

Sulphates.

Jarosite. On the east side of Soda Springs Valley in Esmeralda County, Nevada, at the edge of the foothills by the road to the Vulcan Copper mine, there is quite an amount of a golden-brown micaceous mineral, the origin of which was not * Includes any TiO, and P2O, that may be present.

+ Includes any FeO, calculated as Fe2O3.

On the Serpentine of Montville, N. J., Proc. U. S. Nat. Museum, vol. xxii, pp. 105-111.

investigated. The nearest railroad station is Luning. Some of this material is said to have been used as a pigment for paint at the Midwinter Fair in San Francisco. The mineral was examined by Dr. W. F. Hillebrand, who made the following report:

"The mineral is a basic sulphate of sodium and ferric iron which if freed from foreign matter might agree in formula with jarosite. This is a potassium salt of formula

K ̧O3Fe,0,4SQ,6H,O, or written structurally

[SO,],[Fe(OH),],K, since the water is constitutional, not being expelled below redness."

"The pure crystallized sodium salt does not seem to have been heretofore observed. The mineral contains a small frac tion of a per cent of potassium with nearly 6 per cent of sodium."

Nickel minerals.

About three miles from Candelaria, Nevada, east of the road to Columbus, is a nickel deposit. Specimens sent by Mr. A. C. Dwelle were examined by Mr. E. T. Allen, of the Geological Survey, who found a sulph-arsenide of nickel, perhaps gersdorffite, and a green hydrous magnesian silicate genthite, present.

ART. XXX.-On the Use of the Stereographic Projection for Geographical Maps and Sailing Charts; by S. L. PENFIELD.

[Continued from p. 275.]

Stereographic Map of the United States.-A much reduced copy of the map made by the writer is shown in figure 21, and it should be said regarding it that it was drawn solely for the purpose of making a practical test of the accuracy with which measurements may be made with a scale of miles on such a projection, therefore not much detail is shown. As far as may be told by the eye, the map does not give the impression of being different from others with which we may be accustomed, but, owing to the fact that it is stereographic and so projected as to show minimum distortion, it is believed that it is the best kind of a map, embracing so large an extent of country, which can be made. The cities shown on the map were located from the following data, mostly taken from the Encyclopædia Britannica:

To locate places with accuracy a sliding scale, figure 22, may be used to advantage. It consists of a system of equally spaced, diverging lines, drawn on some transparent material such as tracing cloth or parchment paper. To locate Indianapolis, 39° 47' N., 86° 6' W., for example; on the nearest meridians, 85° and 87°, the approximate latitude is marked with a soft pencil, and the scale is adjusted so that its outer lines cover the meridians at the points indicated. The lines of the scale then subdivide the space between the meridians into eight sections, each corresponding to 15'. The middle line of the scale gives the position of the 86th meridian, the next AM. JOUR. SCI.-FOURTH SERIES, VOL. XIII, No. 77.-MAY, 1902.

dashed line gives 86° 15', and a point a trifle less than half way between them determines a point on the desired meridian, 860 6', which is then transferred to the map by puncturing with a needle point. Through the point thus found a short line is drawn parallel to the meridians, and over it the sliding scale is again adjusted so that its outer lines cover the nearest parallels, 39° and 41°, when the desired latitude 39° 47', is located on the meridian 86° 6' by puncturing through the scale just above the dashed line indicating 45'. Using the method just described, it is probable that places were located on the map within two miles of their true position, the chances being that the average error was less than a mile.

Any straight line drawn through the center of the map, 39° N. 97° W., figure 21, accurately represents the track of a great circle, and the map corresponds to so small a portion of a

22

hemisphere, figure 20, that even near the periphery, where distortion is greatest, a great circle does not differ much from a straight line; this may be tested by applying a straight edge to one of the outer meridians, which show the greatest curvature. Moreover, distortion on the map is so slight that very satisfactory measurements may be made with a scale of miles, and, because of the evident importance of this simple method of making measurements, this property of the map has been very thoroughly tested. The examples chosen include distances over all sections of the map, and they are sufficiently numerous to give a satisfactory average idea of what may be accomplished by such methods of measuring. The adoption of a scale of miles suitable for a given map is a matter deserving special consideration. Taking distortion into account, it is evident that a scale of miles adapted to the center of a stereographic map, figure 21, would be too short when used near the periphery, and vice versa. As, on the other hand, the distortion over the whole extent of the map is not very great, it was assumed that a scale of miles could be found which would give, on the average, satisfactory results when applied to any part of the map. Accordingly the distance from the center of the map, 39° N., 97° W., to a point near the periphery, 37° N., 69° W., was selected as the basis of a scale of miles. The distance measured 35-08cm on the map, and was found by calcula