Изображения страниц
PDF
EPUB

somewhat formal description of the process of their production.

December, 1870.

Since sending the above to press I have subjected plates, coated with "sensitized" collodion to the action of the magnetic field. I had hopes of thus obtaining a physical impress on the plate which would appear on flowing the "developer." Sensitized films on glass and on iron plates were placed over and between the poles of an electro-magnet with cores 17 in. in diameter. Some plates were developed after removal from the magnet, others while under the magnetic action,-with and without the light having acted upon them-but no trace of effect has been detected.

I had also imagined that the magnet's action should have placed the affinities in a more unstable condition, so that the film would rise in sensitiveness after exposure in the magnetic field; but this, also, I could not detect; nevertheless, I have not given up the supposition that some action will be evolved. when more appropriate films, far higher magnetic action and more delicate measures of actinic effect are used. February 13, 1871.

ART. XXXIX. - Notice of a Fossil Forest in the Tertiary of California; by Professor O. C. MARSH, of Yale College.

DURING the visit of the Yale College Scientific party to the Pacific Coast, in October last, several members of the expedition, including the writer, while on their way from San Francisco to the "Geysers," took occasion to examine a locality, a few miles from the route, where a number of fossil trunks of trees had recently been discovered. This point proved to be of much scientific interest, and, since it has received as yet appar ently little or no attention from geologists, it may be well to note the more important results of our investigation.

The locality is situated on a high rocky ridge, in Napa county, California, about five miles southwest of Calistoga Hot Springs, and perhaps ten miles south of the summit of Mount St. Helena. The existence at this place of several petrified trunks of trees was first made public by Charles H. Denison, Esq., of San Francisco, who visited the spot in July last, and, soon after, gave a short account of the discovery in the San Francisco Bulletin. Our party was especially indebted to Mr. Denison for information on the subject, and for accompanying us to the locality during our first visit. The ridge on which the fossil trees were found belongs to the Coast Range series, and forms the divide between the Napa and Santa Rosa valleys.

*

It is about 2000 feet in height, and is mainly composed of metamorphic rocks of Cretaceous age, which are in places, as we ascertained, overlaid, unconformably, by later Tertiary strata, consisting of light-colored, coarse sandstones, and beds of stratified volcanic ashes. This ridge had long been covered with a dense growth of "chaparral," but just before our visit a destructive fire had swept over a portion of it, rendering it comparatively easy to examine a large tract of country which apparently had never been explored.

A careful examination of the locality where the first prostrate trunks had been discovered soon made it evident that those now on the surface had all been weathered out of the volcanic tufa and sandstones, which form the summit of this part of the mountain ridge. Several large silicified trees were, indeed, subsequently found in the vicinity, projecting from the side of a steep bluff, which had partially escaped denudation. Extending our explorations among the mountains for several miles. around, we were rewarded by the discovery of many additional fossil trunks at various points, showing conclusively that this Tertiary deposit contained the remains of an extensive forest of very large trees, which had apparently been overthrown and entombed by some volcanic irruption. Portions of nearly one hundred distinct trees, scattered over a tract three or four miles in extent, were found by our party, and the information we received from hunters and others, familiar with the surrounding country, renders it more than probable that the same beds, containing similar masses of silicified wood, extend over a much greater area.

The fossil trees washing out of this volcanic tufa were mostly of great size, and appeared to be closely related to some of the modern forest trees of the Pacific coast, especially the gigantic Conifers. One of the prostrate trunks examined during our explorations was only partially exposed above the surface, dipping with the strata about 10° to the northward. Its accessible portion, evidently but a small part of the original tree, measured sixty-three feet in length, and, although denuded of its bark and very much weathered, was over seven feet in diameter near its smaller end. On a high summit about a quarter of a mile west of this point, two other large trunks were found, one about five feet in diameter, lying east and west, with thirty feet of its length above the surface. The other rested directly on this, dipping with the strata to the north. The exposed fragments of this trunk indicated that the tree when standing could not have been less than twelve feet in diameter. These two trees had apparently fallen not far from where they were imbedded, as the bark was well preserved, both on the main trunks * Geology of California, vol. i, p. 85.

AM. JOUR. SCI.-THIRD SERIES, VOL. I, No. 4.-APRIL, 1871.

and on the small branches, numerous fragments of which were lying near. Many other trees were found, nearly or quite equal to these in size; and all those examined indicated a very large general growth for the original forest.

All the trees discovered were prostrate, and most of them, after their petrifaction, had been broken transversely into several sections, apparently by the disturbance of the enclosing strata. A majority of the trunks had a general north and south direction, probably due to the course of the current that cov ered them with volcanic material, or perhaps indicating in some cases the position in which they had fallen. Several of the trunks had portions of their roots still attached, and some were evidently much decayed internally and worm eaten before their entombment. All the fossil wood observed was silicified, probably by means of hot alkaline waters containing silica in solution, a natural result of volcanic action, especially when occurring in connection with water, as was evidently the case in the present instance.

The trees closely examined appeared to be all conifers, and in their external characters, especially in the bark, mode of branching, and general habit of growth, most nearly resemble the modern redwoods, still flourishing in the same region. Dr. M. C. White, of New Haven, an authority in such matters, has, moreover, kindly made a microscopical examination of some of the best preserved specimens collected at this locality by the writer, and finds no essential difference between the structure of the fossil wood, and that of existing conifers of the genus Sequoia, to which the redwoods belong.

The beds enclosing the silicified wood appear to contain no other fossils, excepting here and there small fragments of charcoal, and hence their exact age is somewhat doubtful. As they rest unconformably on distorted and metamorphic Cretaceous strata, they are apparently as recent as Tertiary, and will probably prove to be Pliocene. The origin of the volcanic material which covered the forest cannot, perhaps, be definitely ascertained, without a careful examination of the surrounding region. It is not improbable, however, that it came from Mount St. Helena, the nearest volcanic peak, across the now intervening depression. Our party discovered on the western side of the Napa valley, along the base of the ridge, patches of a deposit of stratified tufa and gravel, which was evidently indentical with that containing the fossil trees on the summit. This would seem to imply that the upper portion of the valley had once been filled with these peculiar beds, and, through their denudation, had gradually attained its present proportions. However that may be, the volcanic deposit and its contents is certainly of great interest, even in this land of geological wonders, and is well worthy of a more systematic exploration than we were able to give it in the limited time at our command.

Yale College, New Haven, February 10th, 1871.

ART. XL.-On the Determination of the Alkalies in Silicates by ignition with Carbonate of Lime and Sal-ammoniac; by J. LAWRENCE SMITH, of Louisville, Ky.

IN the following description of a method of separating and determining the alkalies, I aim to give the minutest details. Numerous analyses have given me the experience here presented; and I am convinced that analytical chemists, if they follow out the directions, will not resort to any other known method. If there be a better method it is yet to be discovered. The presence of boracic, hydrofluoric and phosphoric acids in the minerals in no way interferes with the process. Even in silicates soluble in acids, I prefer this method, in common with other analysts, for its ease and accuracy. I made the researches during the latter part of 1852, and the details were published early in 1853.* Since then I have employed the process many hundreds of times, with the most accurate results. Some minor points were not completed satisfactorily until several years after the first notice of the method; these have since been perfected, and I now know of nothing further that is needed.

The purpose of this article is to give all the improvements, with a minute detail of the manipulations, and of the precautions necessary, all of which are simple and easily executed. In the two articles on the subject of alkali determination in minerals published in 1853, the whole subject was reviewed, and it is needless to return to it now. I then considered the processes by caustic baryta and its salts, and by hydrofluoric acid, and also detailed some experiments on the separation of the different alkalies from each other, and on the microscopic examination of the same, etc. It was proved that, after the caustic alkalies, the most powerful agent to attack silicates at a high temperature is caustic lime, a fact not new to chemists. But for the purpose of arriving conveniently by this method at a quantitative determination of the alkalies in silicates, certain methods of manipulation, and facts with regard to quantity of material, admixture, etc. had to be discovered; and in them resides the success of my process-converting the most difficult parts of the analysis of a silicate into the easiest.

The methods of analysis by caustic baryta and by means of its carbonate, are now no longer used, for various reasons fully detailed by Rose in his Analytical Chemistry. The method still extensively employed is that with hydrofluoric acid, pro

*Shortly after my first publication in this country, M. St. Claire Deville made known his method of analyzing the silicates by fusion with carbonate of lime, but the nature of his process and the objects to be arrived at were quite different from those attained in this process.

posed by Berzelius; and when used with the necessary precautions, it has seemed to decompose all silicates; still, according to Rose, there are siliceous compounds that cannot be completely decomposed by hydrofluoric acid.*

Dismissing all criticism, I at once proceed to the method which is the subject of this article, viz: the Decomposition of Silicates by ignition with Carbonate of Lime and Sal-ammoniac. A mixture of carbonate of lime and sal-ammoniac is used in the decomposition simply for the purpose of bringing the caustic lime to act, in a most thorough manner, upon the silicates at red heat.+

Pure carbonate of lime.-The first requisite is pure carbonate of lime. This is made in my laboratory, as follows: Take as good marble (calcite) as can be conveniently found, and dissolve it in hydrochloric acid (it is not necessary that the acid be perfectly pure), add an excess of the marble and warm the solution; to it add lime water or some milk of lime made from pure lime, until the solution is alkaline to test-paper; the lime is added to precipitate any magnesia, phosphate of lime, &c., that may have existed in the marble. Filter this solution and precipitate with carbonate of ammonia, after heating to at least 160° F.‡ The carbonate of lime thus precipitated is to be thrown on a filter and well washed with distilled water. Thus prepared, the carbonate of lime is a dense powder and perfectly pure, or if it contain any impurity, it will be a trace of carbonate of baryta or strontia, which in no way interferes with its use.

Sal-ammoniac.-To obtain this reagent in the most convenient form, take some fragments of clean sublimed sal-ammoniac, dissolve them in water with a gentle heat, filter, evaporate the filtrate over a steam bath or a sand bath, or by means of any other convenient gentle heat, and as the crystals deposit themselves, stir the solution to keep them small; when half or twothirds of the sal-ammoniac is deposited, pour off the liquid without waiting for it to cool, throw on a cotton filter, and dry the crystals at the temperature of the atmosphere. In this way, sal-ammoniac is obtained that can be easily pulverized.

Vessel for the decomposition.-The ordinary platinum crucible can be used for this purpose, and for many years was employed by

*The process used by Deville, in fusing with carbonate of lime, is in most cases better than that by hydrofluoric acid, and one that I should use in preference to all others except the one now under notice.

Chloride of calcium at a red heat will dissolve more or less caustic lime.

This precaution must not be overlooked, as it is desirable to obtain the precipitated carbonate of lime as dense as possible. If the carbonate of ammonia be added to the cold solution, the precipitate, at first gelatinous, will ultimately become much more dense and settle readily; the same is true if the mixture be heated after the addition of the carbonate; but in neither case will it be as dense as when the carbonate is added to the hot solution of chlorid of calcium. The reaction in the analysis is in no way affected by the form of the carbonate of lime; but by using the denser form, the mixture occupies les space in the crucible.

[ocr errors]
« ПредыдущаяПродолжить »