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In the preparation of this table, the original intention was to exclude all observations unaccompanied with any statement of the phenomena. An exception was afterward made in favor of the observations of Bonn and Pulkowa, so that in this respect there is a lack of homogeneousness in the table. The few observations of "apparent contact," have also been added, in the belief that they would not be devoid of interest.

The principal conclusion, to be drawn from the comparison here exhibited, is that there is no discoverable relation between the time of observation on the one hand, and the size of the telescope, the magnifying power, (so it exceed 50 or 60) or the character of the phenomenon on the other. We find the phenomenon of the apparently instantaneous formation of the "black drop" to range from 20h 59m 588 (Le Verrier) to 21h 0m 21.80 (Ventosa). The times of "Vanishing Contact" on which Wolf and André lay so much stress, range from 20h 59m 57s to 21h 0m 29. If we reject Pohl's observation, the range will still be 17. I think if the observations of external contact were collected and compared in the same way, it would be found that their agreement was as good as in the case of internal contact. So far as we have data for judging, these differences would seem to be due to the accidental errors of observation. Their amount in arc may be inferred from the fact that 15s of time correspond to a change of 1" in the relative position of Mercury and the

sun.

I conceive, therefore, that we shall fail if we rely mainly on observations of internal contact. Still, there are two measures by which the reliableness of the determination of ingress and egress may be greatly increased. The first consists in having the observer occupy the entire time of partial ingress and egress in making very careful measures of the distance of cusps with such micrometer as may be best adapted for the purpose. The second consists in bringing the observers at opposite stations together, both before and after the transit, and causing them to make observations on artificial transits with the same instru* Contact seemed to be established simultaneously at several points. + Doubtful. Limbs "certainly in contact by at least that time." AM. JOUR. SCI.-SECOND SERIES, VOL. L, No. 148.—JULY, 1870.

ments employed in observing the transit of Venus, in order to determine what correction should be applied to the observations of one to make them comparable with those of the other. It would be a comparatively simple operation to erect an artificial representation of the sun's disk at the distance of a few hundred yards, and to have an artificial planet moved over it by clockwork. The actual time of contact could be determined by electricity, and the relative positions of the planet and the disk by actual measurement. With this apparatus it would be easy to determine the personal errors to which each observer was liable, and these errors would approximately represent those of the observations of actual transit.

Still, it would be very unsafe to trust entirely to any determination of ingress or egress. Understanding the uncertainty of such determinations, the German astronomers have proposed to trust to measures with a heliometer, made while the planet is crossing the disk. The use of a sufficient number of heliometers would be both difficult and expensive, and I think we have an entirely satisfactory substitute in photography. Indeed, Mr. De la Rue has proposed to determine the moment of internal contact by photography. But the result would be subject to the same uncertainty which affects optical observations-the photograph which first shows contact will not be that taken when the thread of light between Venus and the sun's disk was first completed, but the first taken after it became thick enough to affect the plate, and this thickness is more variable and uncertain than the thickness necessary to affect the eye. We know very well that a haziness of the sky which very slightly diminishes the apparent brilliancy of the sun, will very materially cut off the actinic rays, and the photographic plate has not the power of adjustment which the eye has.

But, although we cannot determine contacts by photography, I conceive that we may thereby be able to measure the distance of the centers of Venus and the sun with great accuracy. Having a photograph of the sun with Venus on its disk, we can, with a suitable micrometer, fix the position of the center of each body with great precision. We can then measure the distance of the centers in inches with corresponding precision. All we then want is the value in arc of an inch on the photograph plate. This determination is not without difficulty. It will not do to trust the measured diameters of the images of the sun, because they are affected by irradiation, just as the optical image is. If the plates were nearly of the same size, and the ratio of the diameters of Venus and the sun the same in both plates, it would be safe to assume that they were equally affected by irradiation. But should any difference show itself, it would not be safe to assume that the light of the sun encroached equally upon the dark

ground of Venus and upon the sky, because it is so much fainter near the border.

If the photographic telescope were furnished with clock-work, it would be advisable to take several photographs of the Pleiades, both before and after the transit, to furnish an accurate standard of comparison free from the danger of systematic error. There is little doubt that if the telescopes and operators practice together, either before or after the transit, data may be obtained for a satisfactory solution of the problem in question.

To attain the object of the present paper, it is not necessary to enter into details respecting choice of stations and plans of observation. I have endeavored to show that no valuable result is to be expected from hastily-organized and hurriedlyequipped expeditions; that every step in planning the observations requires careful consideration, and that in all the preparatory arrangements we should make haste very slowly. I make this presentation with the hope that the Academy will take such action in the matter as may seem proper and desirable.

ART. XI.-On the Geology of Eastern New England; by Dr. T. STERRY HUNT, F.R.S. (From a letter to Prof. JAMES D. DANA.

WHEN, more than twenty years since, my attention was turned to the geology of New England, there was no evidence of the existence between the old gneisses of the Adirondacks and the coal measures, of any other stratified rocks than those of the Huronian series, and the New York system, from the Potsdam formation, upward. It is true that Emmons had, before that time, maintained the presence, in western Vermont and Massachusetts, of a system of fossiliferous sediments, lying unconformably beneath the Potsdam, but the evidence up to this time adduced with regard to these so-called Taconic rocks, has failed to show that they include any strata more ancient than the Potsdam, while most of them are certainly younger. The researches of Sir William Logan, up to 1848, had led him to refer to a period not older than the Lower Silurian the crystalline sediments of the Appalachian region of Canada, between Lake Champlain and Quebec. These form a chain of hills, the continuation of the Green Mountains, and were found by him to be followed immediately, to the southeast, by more or less calcareous and somewhat altered strata, associated with Upper Silurian fossils, and succeeded, across the strike, near the sources of the Connecticut River, by a series, several miles in breadth, of micaceous schists and quartzose strata, occasionally containing

chiastolite, garnet and hornblende. These two series of rocks, extending from the base of the Green Mountains to Canaan on the Connecticut, it was suggested by Sir William Logan, in his Report on the Geological Survey, 1847-1848, might be the altered representatives of the rocks of Gaspé, including the Lower Helderberg group, and the succeeding members of the New York system to the top of the Chemung. I then, as now, conceived that these micaceous and argillaceous schists, often holding garnets and chiastolite, were identical with those which make so conspicuous a figure in the White Mountains, and elsewhere in Eastern New England, and when, in 1849, I laid before the American Association at Cambridge, the results of the Geological Survey of Canada (this Jour., II, ix, 19), suggested that to the Gaspé series, as above defined, "may perhaps be referred, in part, the rocks of the White Mountains." ley, subsequently, in 1860 (Proc. Philad. Acad. Nat. Sciences, page 363), adduced many reasons for believing that the rocks of these mountains might be strata of Devonian age. In the large geological map of Canada and the northern United States, lately published by Sir William Logan, no attempt is made to delineate the geology of New Hampshire, but the rocks in question, to the north of the United States boundary, are represented as Upper Silurian, with the exception of a belt of the Quebec group, which has been recognized in that region.

*

Les

In fact, the schists and gneisses of the White Mountains are clearly distinct, lithologically, from the Laurentian, the Labradorian and the Huronian, as well as from the crystalline rocks of the Green Mountains, and from the fossiliferous Upper Silurian strata which lie at the southwestern base of the Canadian prolongation of the latter. Having thus exhausted the list of known sedimentary groups up to this horizon, it was evident that the crystalline strata of the White Mountains must be either (1) of Devonian age, or (2) something newer (which was highly improbable); or (3) must belong to a lower and hitherto unknown series. In the absence of any proof, at that time, of the existence of such a lower system, the first view, which referred these strata to the Devonian period, was the only one admissible.

*In this connection should be recalled the views put forth in 1846, by Messrs. H. D. and W. B. Rogers, in a paper on the Geological Age of the White Mountains, (this Journal, II, i, 411). They there, for the first time, pointed out that the great mass of these mountains consists of more or less altered sedimentary strata, which, upon the evidence of supposed organic remains, they referred. with some little doubt, to the Clinton division of the Upper Silurian. In 1847, however, they announced that the supposed fossils, on which this identification had been founded, were not really such, (this Journal, II, v, 116). Future explorers may, it is hoped, be more successful, and yet discover among the strata of the White Mountains evidences of organic life, probably of primordial Silurian age.

When, however, further investigation showed that the great and progressive thickening which takes place in the paleozoic formations from the west, eastward, is not confined to the augmentations of existing subdivisions, but includes the intercalation of new ones; when the few hundred feet of typical Potsdam sandstone in New York are represented in Vermont, Quebec and Newfoundland, by thousands of feet of strata lithologically very unlike the type; while the Quebec group, not less in volume, appears representing the beds of passage between the Calciferous and Chazy divisions of New York, we begin to conceive that conditions of sedimentation, very unlike anything hitherto suspected in the west, prevailed to the eastward. When, moreover, we find widely separated areas of Labradorian and Huronian rocks,-remaining fragments of great series, resting upon the Laurentian, from Lake Huron to Newfoundland, we get evidences of a process of denudation in past ages, not less remarkable than the sedimentation.

My observations of last year have led me to a conclusion, which had previously been taking shape in my mind, that there exists above the Laurentian, a great series of crystalline schists, including mica-slates, staurolite and chiastolite-schists, with quartzose and hornblendic rocks, and some limestones, the whole associated with great masses of fine-grained gneisses, the so-called granites of many parts of New England. The first suggestions of this were given me by the observation of Dr. Bigsby, confirmed by specimens since received from the region, that there exists to the northwest of Lake Superior, an extended series of crystalline schists, unlike the Laurentian, and resem. bling those of the White Mountains. I have already called attention to this resemblance in a review of the progress of American Geology, in 1861 (this Jour., II, xxxi, 395). It was contrary to my notions of the geological history of the continent to suppose that rocks of Devonian age could, in that region, have assumed such lithological characters, and I was therefore led to compare these rocks with a great series of crystalline schists, abounding in mica-slates and miçaceous limestones, which occupy considerable areas in the Laurentian region in Hastings county, to the north of Lake Ontario. The distribution of this series has been traced out by Mr. Vennor, who, in 1869, was able to show that, although much contorted, it rests unconformably upon the old Laurentian gneisses, while it is, at the same time, overlaid by the horizontal limestones of the Trenton group. This intermediate series, which attains a thickness of several thousand feet, is terminated by calcareo-micaceous schists, in which Eozoon Canadense has been found, both in Madoc and in Tudor. In these localities, as shown by Dawson and Carpenter (this Jour., II, xliv, 367), the calcareous

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