the oscillations of temperature have been generally marked around our globe and have not been the result of local geological disturbances. That the oscillations were slow and progressive is shown by the distribution of the species of plants in both the following formations. In the Miocene of Vancouver the Proteineæ are dominant. It has also palm trees and Salisburia, all tropical plants, and most of the species are without relation to the plants now living on this continent. In the Pliocene of Tennessee the Proteineæ appear still abundant and the flora finds its relatives in the southern shores of Florida and on the islands of the Gulf of Mexico. The Post-pliocene of the Mississippi near the mouth of the Ohio river, and even above it, has the same species of plants as are now found along the shores of the Atlantic, in the southern states. We have thus apparently a steady decrease in the temperature from the Miocene to the Post pliocene of the Mississippi. From this it appears to follow that the chalky banks of which the true geological position is still uncertain, ought to be regarded as anterior in origin to the Drift. For it is probable that if they had been deposited after or at the time of the ice period, the distribution of the plants would show a colder climate rather than the climate of our southern shores.

ART. XL.-On Bornite from Dahlonega, Georgia; by Dr. C. T. JACKSON.

BORNITE occurs in Field's gold mine, in Dahlonega, Georgia, in a vein of quartz, associated with native gold and some auriferous iron pyrites, in hornblende slate rocks, bordering the Chestertee River.

The mineral is found in thick foliated masses, having a crystalline structure probably hexagonal, though not perfectly defined. The masses are from half an inch to one inch in diameter, and they split like talc and mica into thin plates, quite as readily as talc.

Its lustre and color, are like those of highly polished steel. It is flexible, sectile, and soils the fingers like plumbago or molybdenite. Its streak on porcelain is metallic, or near the color of the pulverized mineral. Hardness between that of gypsum and calcareous spar, but nearer to the former, say H. 2.25. Density 7-868. Before the blowpipe on charcoal melts, giving out white fumes, which have the odor of selenium, leaves a white deposit on the cold charcoal, and near the bead a ring of yellow color, and a little metallic bismuth is obtained.

=

This, cupelled, gives a little gold. In an open glass tube no smell of sulphur observed; a white smoke fills the tube, and condenses in it. Heated, this deposit forms little yellowish globules. At the lower extremity of the tube, a fused metallic mass remains adherent to the glass. A little brown sublimate is mixed with the sublimed telluric acid, and is selenium.

One gram of this mineral selected with care, to avoid all admixture of pyrites, was analyzed and the following results were obtained:

The bismuth was separated from the nitric solution by carbonate of ammonia, and was several times redissolved and precipitated anew, to free it from all traces of telluric acid. It was then converted by heat, in a porcelain crucible, into oxyd of bismuth. The whole of the washings and the filtrate, mixed, was evaporated to small bulk, and the nitric acid was decomposed and removed by repeated additions of chlorhydric acid and heat until no more chlorine was given off. Then the solution was brought to near neutrality by ammonia and a current of sulphurous acid gas was passed through it, until all the tellurium appeared to be reduced. It was then filtered and washed with water saturated with sulphurous acid, and the filter which had been properly tared was weighed, when dry, at 212° F.

On addition of a solution of sulphite of ammonia to the filtered solution, and allowing it to stand for forty-eight hours, more metallic tellurium subsided, and was collected in a tared filter and the amount was added to that first obtained. Standing twelve hours longer this solution gives no more deposit, though it smells strongly of sulphurous acid gas.

The selenium was determined on a separate sample, of one gram of the Bornite, by converting the selenium into selenic acid, by prolonged digestion in nitro-muriatic acid. Then the selenic acid was precipitated by nitrate of baryta, as seleniate of baryta. It weighed 0.042 gram.

By separate experiments, it was ascertained that no sulphuric acid existed in the solution of the Bornite; hence no sulphur was present.

Boston, March 12, 1859.

ART. XLI.-Geographical Notices. No. VII.

PHYSIOGRAPHY OF THE ISTHMUS OF CHOCÓ, NEW GRANADA. By ARTHUR SCHOTT.*-The transit line across the Isthmus of Chocó, New Granada, has been lately re-surveyed, from ocean to ocean, in the neighborhood of the seventh parallel of north latitude, by a party acting under the authority of the U. S. government. The following facts were collected in connection with the field work of the topographical party, under the immediate orders of Lieut. N. Michler, U. S. Topog. Engineers, coöperating officer of the expedition. A more general account of the results of the survey was given in this Journal, November, 1858.

The entire length of the route surveyed is about 160 miles, belonging entirely to the torrid zone, as its greatest elevation does not exceed 1000 feet above the level of the sea. By its physical features this line is divided into two distinct portions, differing from each other both in extent and in meteorological condition. The western or Pacific slope, being only 15 miles long, has an almost constantly dry climate, which appears to be peculiar to the whole extent of the Pacific coast. The eastern or Atlantic slope, 147 miles in length, situated under a sky perpetually clouded, is drenched for eight or nine months of the year by daily rain, more or less heavy. Its atmosphere at the same time is kept in a state of perpetual oscillation by neverceasing electric changes. Along this portion of the line, the features of the country are decidedly aquatic, varying according to the hypsometrical subdivisions.

From the level of the salt water in the Gulf of Urabá to its marshy uplands and scarcely ventilated mountain forests, every kind of lowland" is represented,-mangroves, lagoons, everglades, forest swamps and ever-shady uplands.

To facilitate a more detailed examination of the country the following synoptical table is submitted: I. Mangroves and tidewater lagoons. II. Atrato levees. III. Everglades and overflow of the Atrato. IV. The palisades. V. The lowland. VI. The tableland. VII. The Cordillera or Divide. VIII. The alluvium. IX. The mangroves. X. The beach (La Playa).

Not only is the Atlantic slope found at first sight to exceed very much in extent the western or Pacific slope (a feature which applies generally to the whole of the continent,) but also each of the topographical subdivisions mentioned is much more developed on the eastern side than on the western. Throughout almost the entire route, the aquatic character already referred to is distinctly marked in its fauna and flora. A more detailed review

* Communicated for this Journal by permission of the U. S. Navy Department.

commencing on the Atlantic shore in the Gulf of Urabá may be added to the table just given.

Section I. Mangroves and Tidewater Lagoons.-Among plants we find prevailing here Rhizophoreæ, aquatic Gramineæ, Polygonaceæ, Aroideæ, lacustric Palmacea and Musaceæ, all which by habit correspond to amphibious reptiles, to Natatores and Grallatores among the birds or to the cetaceous Manati, and to fluviatic Cavidæ like the Agouti and Lancha (Dasyprocta and Hydrochorus). As occasional forms of animals we here find the red roaring monkey (Mycetes seniculus), and of birds two forms related to Sturnidæ, which suspend their nests from the branchtops of the mangroves, thus making them inaccessible to their enemies. To these may be added a genus of Psittacidae in the shape of the psittaceous Macao. These birds leave their home in the more elevated regions on the eastern shore of the gulf, to follow their daily sport all over the Atrato delta, which in the evening they leave again. Nearly related, anatomically, to the Scansores are the Halcyonidæ, and we may rightfully consider them as icthyophagous climbers. They are represented by three or four distinct species, and form a characteristic type along the whole line.

This section, of an extent of about twelve English miles by way of the river, may be characterized geologically as floating alluvium, which is covered by a low but densely interwoven arboreal vegetation.

Section II. The Atrato Levees.-Where the surface of the country rises up to and above high-water mark, plants of more terrestrial habits are added to the former. Thus Leguminosa with their suborders Cassie and Mimoseæ, also Malpighiaceae, Malvaceæ, Euphorbiaceæ, Apocynaceæ, Margraviaceæ, Lecythidaceæ, Melastomaceæ, Bignoniaceæ, and others, are met with under a most diversified generic display. While these orders are so varied in habit, and the division of Scandentes is a prevailing type, we find a close analogy to them in the members of the fauna inhabiting this ground. Among the reptiles we observe the Iguano and the Basilisc, both of arboreal habits and only occasionally taking to the water. The Quadrumana generically increase. Mycetes Beelzebub and Pithecia leucocephala (the Zambo and the Mono cara blanca) appear with the roaring monkey. Here also the low form of Bradypus finds a safe and solitary retreat. Among the birds, the Scansores, the most appropriate form for this section of country, are represented by almost every variety. The carpophagous Psittacidae, the entomophagous Picidæ, and the sarco- or at least oö-phagous Ramphastidæ are leading forms, together with the ichthyophagous

SECOND SERIES, VOL. XXVII, No. 81.-MAY, 1859.

Halcyonidae and the Anhinga which is a Pelicanid of arboreal habit.

Keeping step with the gradual elevation of the country the fauna and flora increase. Musacea and Amomacea, with their peculiar maximum development of chlorophyllum, may be considered as equivalents of the two herbivorous pachyderms, which seem to feed on them; they are the tapir and the peccari (Danta and Sajino of the natives). Corresponding to them is the Manati, which is said to abound through this section as it does also within the delta.

The section of the Atrato levees in its geological character is, if the expression is admissible, truly amphibious, for during eight or ten months it is thoroughly swamped by the overflow of the Atrato.

The growth of trees upon it is nevertheless very heavy, and the traveller here meets those well known mighty leguminous giants in company with mammoth forms of Bombax ceiba, and also with Cedrela, Carolinea or a Tecoma. They maintain a prominent stand upon the banks of the river or arrange themselves there in closed ranks as an impenetrable phalanx.

Here the river flows in solitary grandeur, reflecting from its mighty sheet of water an ocean of light and giving freedom to the aerial currents, which show their effects in the surpassing beauty of a tropical flora. Such is the difference, where light and air have access, that even the plague of mosquitos and the lurking effects of sickly miasms, which have their dominion within the enclosures of the swampy forests, disappear upon the openings of the river.

It is a peculiarity of the Atrato that it forms, throughout its course of nearly sixty miles from the head of its delta up to the mouth of the Sucio and still higher up, but one bed, within which it keeps collected the whole body of water, bordered by vertical banks and having an average depth of over fifty feet.

The Atrato levees are scarcely inhabitable. Even the Indians and few Zambos remain upon them only occasionally for temporary fishing and hunting. The only settlement is found at Sucio, and this is but a mere trading station for the Atrato navigators and a shipping depot for the collected raw material of caoutchouc and ivory-nuts. A few natives only remain in this place to profit by raising plantains, bananas and other fruits, Indian corn, calabassas and cacao, which latter prospers here and is of superior quality.

Section III. The Everglades.-In leaving the Atrato levees through the mouth of the Truandó a relapse of level is reached, which leads for about 18 or 19 miles through a region of everglades. They form on both sides of the Atrato the recipients