or more.

A steep chasm or channel extends from the top of the ridge to its base, and is partially filled with rocks and the debris of the vein. Solid blocks of the ore were found with this accumulation, having been broken out from the vein above; one of them was twenty-seven inches long and sixteen to eighteen wide.

The ore is associated with quartz, and where it has decomposed, an abundance of antimony ochre is found, together with crystals of selenite. Specimens of quartz traversed by long prismatic crystals of the ore were obtained.*

SALT.-Salt is found in small quantity as an incrustation or efflorescence on the soil along streams or on the margins of ponds in nearly all parts of California. It appears to be most abundant in connection with the tertiary strata and in the streams that flow from them. It is doubtless the fact that a great part of the incrustations called soda, consist principally of common salt.

Tulare Co.-Cañada de las Uvas.-There is a small shallow lake near the central part of this Pass fed by springs and streams from the adjoining valleys and ridges which are partly of tertiary strata. During the suminer season the water of this lake evaporates, and its bed becomes covered with a white crust of salt which glitters in the sunlight like a field of snow.

Taheechaypah Pass.-A lake of a similar character to the one just described is found in one of the elevated valleys of the Sierra Nevada near this Pass. At another locality in that vicinity and near the margin of the Great Basin, salt occurs in a thick bed, from which over one hundred mule-loads have been taken, and carried to the Tejon Indian reservation for the use of the Indians. This salt is perfctly white and amorphous, being reduced to a fine powder by simple pressure. It is sufficiently pure for table use. Dry salt lakes are also found near the termination of the Mojave river in the Great Basin, and at many other places throughout Southern California.

Los Angeles Co.-Salt is now manufactured in large quantity from sea-water by solar evaporation on the coast near Los Angeles.

Lower California.-A dry salt-lake has been discovered about 250 miles south of San Diego and near Marguerita bay. It forms a thick bed and is very pure, being well crystallized in large hopper-shaped crystals. It is reported that the locality has been purchased by capitalists and that the salt is being shipped from there in large quantity.

GYPSUM.-Transparent plates of selenite are common in the soft unconsolidated tertiary strata in various parts of the state. At some localities it forms seams or beds several inches thick lying conformably with the stratification. In Tulare Co., at Ocoya

* A more detailed notice of this locality will be found in the author's Preliminary Geological Report accompanying the Report of a Reconnoissance and Survey in California, by Lieut. R. S. Williamson. House Doc., 129. 1855.

creek these transparent plates are found in the Miocene strata; some of them are combined with the fibrous variety, and form beautiful cabinet specimens. Good crystals are also found in this county, at the antimony localities. Thin transparent plates are numerous in the Miocene strata bordering the Colorado Desert, and on the borders of Carrizo creek they are found lying loose upon the surface where the strata have been worn away by the rains. Seams of gypsum are numerous in the tertiary strata of Benicia.

Wherever I saw the gypsum in the tertiary strata, it appeared to have been formed by the infiltration of sulphates and their decomposition by the lime of the beds.

-

BITUMEN. The occurrence of bitumen springs in the Coast Mountains has been noticed in the writer's preliminary Report.*

Near the Pueblo de los Angeles, there is a large pond or lake of the bitumen about one quarter of a mile in diameter. Its central portion is soft and semifluid but the outer parts are hardened by exposure. The material is much used for covering roofs, and at Monterey I saw a good basement formed from its mixture with sand and gravel.

SULPHUR. Very interesting specimens of sulphur can be obtained at the Geysers or hot springs in Napa Valley. It occurs in crusts or lining fissures in the soil and tufaceous deposits around the springs, and is in small crystals, forming drusy surfaces.

BERYL-Tuolumne Co.-Small and well formed hexagonal crystals having the hardness and color of beryl have been obtained from the Tuolumne river three or four miles from Jamestown. The specimens that I saw were apple-green, and one of the smallest was emerald-green and transparent. The largest crystal was nearly of an inch in diameter and terminated at both ends with the planes R, and -, as in tourmaline. I was unable to retain the specimens for further examination.

TOURMALINE.-San Diego Co.-Black tourmalines of unusual size (from six to eight inches in diameter) occur abundantly in the huge feldspathic veins that traverse the granite ridges bordering the elevated valley of San Felipe, in the mountains between San Diego and the Colorado desert. These crystals are not perfect.

FELDSPAR.-ORTHOCLASE.-San Diego Co.-Good crystallizations of this mineral can be found in the granite veins near the road between Santa Isabel and San Pasquale. They are associated with tourmalines and garnets.

ANDALUSITE, Mariposa Co.-This interesting mineral was found in great abundance in a conglomerate that caps the hills along the Churchillas rivers (San Joaquin valley) at the crossing of the road leading to Fort Miller.

* See also this Journal, [2] xix, p. 483.

Very fine crystals of unusual size occur in the gravel along the bank of the stream. I picked up several that were two inches long and three quarters of an inch in diameter. They have a delicate pink or rose color and some of them are translucent. The peculiar tesselated appearance displayed in a cross section of crystals of this species, is exhibited by these specimens in a beautiful manner.

CALCITE-Crystallizations of this mineral are found at the Quicksilver mine, (New Almaden), at the Pass of Jacum, San Diego Co, and on the surface of the Colorado desert north of Carrizo creek, where some transparent crystals were picked up. It also occurs in beautiful stalactites and delicate crystals in the great cave in Calaveras Co.

ART. IX.-Analysis of Idocrase from Ducktown, Polk Co., Tenn.; by J. W. MALLET, Ph.D.

THE specimen of this Idocrase examined occurred among some other minerals from the Ducktown copper mine, in thin bladed crystals imbedded in a mixture of copper and iron pyrites. The crystals were from half an inch to two inches in length, about half an inch in width, and not more than an eighth of an inch thick. They were longitudinally striated, and presented scarcely any planes, lateral or terminal, suitable for measurement by the reflecting goniometer. The existing planes however seemed to be 0, 1, 0, 1-∞, and co-co, the last being most developed, and the others very small. The mineral was nearly colorless, exhibiting but a slight tint of brownish or yellowish green, and was subtransparent. H. =65; sp. grav. 3·359, taken with fragments. Analysis gave the following results, a little copper present being calculated as copper pyrites (derived from the gangue), and the necessary amount of iron subtracted from the total quantity of oxyd of iron weighed :

Atoms.

We have here a slight excess of alumina over the amount required by the formula 3RO, SiO3+Al2O3, SiO3, and a slight defect of protoxyds. The fragments of mineral were selected with great care, and contained no visible particles of ganguehence the copper found in the analysis may perhaps, as in Cyprine, belong to the Idocrase itself, in which case the protoxyds would be increased by the copper and iron calculated above as copper pyrites.

ART. X.-Observations on Binocular Vision; by Prof. WILLIAM B. ROGERS.

THE remarkable discoveries of Prof. Wheatstone in relation to binocular vision, and the observations of Sir David Brewster, Prof. Dove and others in the same field, must be regarded as among the most important contributions ever made to the science of physiological optics. Nor has the knowledge of these beautiful and curious results been confined to the circle of scientific inquirers. The diffusive intelligence of the age has converted the stereoscope into a popular source of instructive recreation. much as has been done, especially by Wheatstone and Brewster, towards forming a true theory of binocular vision, it must be apparent to all who are familiar with their labors that these observers differ in some important particulars, as well in their account of the phenomena as in their mode of interpreting them.

But

In the following pages I propose to record the substance of a series of observations, begun some years ago with the view of repeating and varying the experiments, and if possible testing the conclusions previously made known in regard to this interesting subject. It will be seen that the modes of observation on which I have fallen are in some respects new, and have the advantage of great facility, and that the results and the inferences which I have drawn from them, lead to views of the process of binocular vision differing in some respects from those of either of the eminent philosophers to whom I have referred. To form a theory of this process in all respects satisfactory may require further observations and greater subtlety of metaphysical interpretation, but I trust that the facts and conclusions which I have to present will throw light on some obscure points of the subject and will perhaps disclose features of it which have not hitherto been noticed.

PART FIRST.

1. On the place in which we perceive objects that are binocularly combined.

The learned Dr. Smith of Cambridge, in his "Complete System of Optics," p. 388, describes an experiment in which by binocular vision he united into one long perspective line the legs of a pair of compasses partially opened and held in a particular position directly before him, and he ascribes the apparent union to the fact that the pictures of the corresponding parts of the two legs fall upon the same points of the retinas as that of the object to which the eyes are directed." It is therefore clear that this writer apprehended the primary Law of binocular combination very much as it is at present understood. Although mistaken in his account of part of the experiments and in other ob

servations in binocular vision, he is I think entitled to the credit of having first perceived a chief condition of these phenomena.

But the law of the apparent place of objects binocularly combined is so fundamentally important as to call for other and better modes of proof. One such, remarkable for its completeness, has been described by Sir David Brewster (Phil. Mag., 3d Series, Vol. 24), and I may be permitted to add the following simple experiments for the same purpose which I have found very satisfactory and easy of execution.

1.

On a light strip of wood about 3 feet long and 4 inches wide, two lines are to be drawn diverging from one end at such an angle that their terminations at the other end shall be a little nearer one another than the pupils of the two eyes (fig. 1). Three steel pins each about 2 inches high and surmounted by round heads th inch in diameter may then be fixed perpendicularly in the wood, one of them at the angular point, and the two others at points of the lines not nearer than the limit of distinct vision. The instrument is to be brought close to the face, in such position as to enable us at the same time to cover r by a in the right eye, and by b in the left. If now we direct our attention to the remote pin r we see what appears to be another pin thicker and taller than r, coinciding with it or very near it. This is the binocular resultant of a and b as seen by the two eyes converged to r.

To obtain this effect in the most satisfactory manner the unpractised observer will find it useful to conceal the surface of the board by interposing a low screen between a b and the near end of the instrument, or he may effect the object by depressing the remote end until the lower part of a and b ceases to be visible. If, while directing the eyes steadily towards r so as to perceive the resultant image of a and b, we gently vibrate the instrument in its plane around a point midway in a b, we see the remote pin passing across the position of the compound image, sometimes behind and sometimes in front of it, but always very near. This curious result never fails to show itself, if we are careful to direct the eyes steadily upon the image while moving the instrument, and it gives us a clear impression of the place of the resultant as coincident with the point to which the optic axes are converged. It is important to observe that while viewing the compound image in this experiment, we see, on the right of it, the image of