power of resistance in the arch is strikingly illustrated; for while the crest itself remains perfect, the same strata have been much disrupted and displaced as they pass out of the arch and change their curvature. The upper laminated portions are exceedingly wrinkled, and the heavy beds broken and doubled. Under these circumstances of displacement but one or two of the heavier beds of argillite are exhibited above the grade of the railroad; and while these, through scales or plates retain marks of stratification so as to expose their true character, they have been sharply folded and collapsed, and by their nearly vertical position have the appearance of intrusive masses. The central portion of the cutting-that is, the undisrupted strata-is traversed by a system of parallel joints which the disrupted portions are without. Several segregated veins of quartz with limonite occupy the divisional planes of stratification.

Other sectional cuttings in the several mines or quarries display more or less completely the stratification and undulations of the hematites in common with the Huronian schists of this region, and show conclusively, if further testimony were requisite, the existence of the iron ore under the same conditions of deposit and secondary modification as the schists with which they are associated. In the Jackson mine, for instance, occur two bosses or short anticlinal folds, while in the Lake Superior mine a perfect synclinal flexure is exhibited.

Beds of specular conglomerate are of frequent occurrence throughout the Iron region of Northern Michigan, consisting of a paste of specular peroxyd of iron, through which are disseminated fragments of jasper, and rounded pebbles of specular iron ore which usually differ from the paste in texture, a difference very perceptible among ores of any one class even within narrow limits of distribution. These conglomerates not unfrequently resemble breccia in the angularity of the jasper fragments which they contain; but the pebbles of specular peroxyd, although sometimes obscure in a matrix of the same material, commonly serve to indicate the detrital origin of these beds. That they are derived from local detritus is evident from the fact that the jasper fragments are not rounded, while the particles of softer specular iron ore are worn but slightly. They seem to be of littoral formation and to have been derived from dismembered and crumbled deposits of successive laminæ of jasper and iron ore-similar to those deposits distinguishable in the bosses of the region. The specular conglomerate invariably exists under circumstances of true bedding, and is traversed by parallel joints splitting the imbedded pebbles. It occurs interstratified with talcose and argillaceous schists quite as regularly as the homogeneous ores. As would naturally be expected, the specular conglomerates, owing to their enduring composition, have re

AM. JOUR. SCI.-SECOND SERIES, VOL. XXXIX, No. 117.-MAY, 1865.

sisted erosion more effectually than the purer schists, and are preserved in elevated outliers, as seen particularly at the New England mine, and in the N. W. quarter of Section 21, T. 47 N., R. 27 W. A specular conglomerate uncontaminated with any considerable portion of jasper, forms the bulk of the schists at present wrought by the Lake Superior mine. These are jointed obliquely, and are cleavable at right angles, to the stratification. Specular schist often occurs charged with detrital quartz, or sand, thus differing from the conglomerates only in its external properties, while it is analogous to them in attitude and actual composition. Schists of this description are intercalated with the conglomerates in Section 21 above noticed, and in Section 26 of the same township are found underlying the laminated beds which crown the anticlinal arch.

An estimation of the thickness of the ferriferous series is attended with no little difficulty at present owing to the absence of any entire sectional cutting. The Jackson mine presents the best data for an opinion upon this point, although its workings expose neither the base, nor uppermost members of the series. It is upon the northern dip of a steep anticlinal, and the ground of the quarry is some 500 feet wide, the excavation having been entirely within the series. At this mine the range of thickness exceeds one thousand feet. Individual thicknesses of specular schist without the intervention of other schists, are upwards of 150 feet. Some idea of the massiveness of a homogeneous bed of specular schist may be had from the fact that in the month of September last 3500 tons of workable ore were thrown down at this mine in one blast, for which 11 kegs of powder were used.

It will be observed that while the smaller plications furnish the most available and complete evidences of the stratigraphical conditions of the ferriferous schists, every exposure of them in quarries or natural outcrops, conveys the same character of evidence, but upon a scale far more extended, and generally requiring allowance for superficial vicissitudes, and a large degree of denudation. Even if space permitted, I conceive it to be unnecessary to multiply instances of this evidence. It has been shown that the iron ores of the Huronian series in Michigan are essentially schists and heavy-bedded strata in which none of the phenomena of aqueous deposits formed by precipitation from water on the one hand, or by detrital accumulation on the other, are wanting. They exhibit not only stratification, anticlinal and synclinal folds, but are invariably traversed by systems of joints, and at many points exhibit a perfect slaty cleavage.

The intimate connection between the greenstones, hornblende rocks, and aluminous and magnesian silicated schists of the ferriferous series, has already been indicated in general terms, these rocks not only alternating with, but passing into, each other—a

conjunction commonly existing wherever bedded greenstones, and isolated silicates occur together as sedimentary products of the decomposition of compound silicated rocks. The peculiar green slates which have a large development in this series, are intermediate in composition between clay slate and hornblende slate, and together with the talcose and chloritic slates, with which they are interstratified, are probably products of such a decomposition in the wet way of the same crystalline sediments which entirely or less undecomposed have gone to form those greenstones which constitute members of the same series. Effervescence with acids of some of the green slates in common with many of the greenstones of the series, together with the presence of a large amount of lime and magnesia, was pointed out by Messrs. Foster & Whitney, as indicating the characters of a pulverulent greenstone-the schalstein of German geologists." The presence of carbonate of lime in many of the schistose rocks of this region must be regarded as generally due to the decomposition of silicate of lime in the silicates of the primary crystalline sediments whence they are derived, although in a few special instances it may have been derived by the subsequent conversion of greenstones into schalstones, or by infiltration from superincumbent calcareous strata. Chemical reactions in crystalline sediments resulting from the disintegration of crystalline silicated rocks, and operated upon by carbonated waters, are amply capable to have produced the lithological conditions of augitic rocks, clay-slates, schalstone, and other schists, together with the oxydized ores of iron intercalated with greenstone among the ancient crystalline rocks of North America as well as Europe-as shown by Bischof" in pronouncing the Neptunian characters of these rocks. From a stratigraphical point of view, while evidence is elsewhere often obscure, the Huronian greenstone, schists and iron ores of Northern Michigan, in the absence of close attention to their special chemical conditions, exhibit sedimentary and metamorphic phenomena adequate to render quite untenable, it is believed, the theory of the exotic character of any portion of them.

New York, Dec. 19th, 1864.

12 Report of the Geology of the Lake Superior Land District, ii, 17, 93.

13 Bischof, Elements of Chemical and Physical Geology, London, 1854-59, vol. iii, chap. 55. Volger, Studien zur Entwicklungsgeschichte der Mineralien. Zurich,

1854.

ART. XXXVII.-Astronomical Photograyhy; by LEWIS M.

RUTHERFURD.

My present observatory is a circular brick building of twenty feet internal diameter, with a light revolving roof supported on twelve wheels which are fixed to the stone coping of the walls.

The opening, two feet wide, extends from side to side with simple shutters, which, when elevated on the weather side, serve to prevent the wind from blowing into the observatory and shaking the telescope. Opening from the west side of the Equatorial dome is a small transit apartment with computing room attached. This observatory is in the garden of the house where I reside. The transit is 189 feet N.W. from the Second Avenue, and 763 feet N.E. from Eleventh street. It was erected in the summer and autumn of the year 1856. The equatorial, by Fitz, is a very substantial instrument, having circles divided on silver 18 and 20 inches in diameter.

The objective is of 11 inches aperture, and fourteen feet focal length, and was corrected for figure by myself after the methods and directions of Mr. Fitz. It is a fine glass, capable of showing any object which should be seen by a well corrected objective of those dimensions.

The observatory is low and therefore cannot reach any object near the horizon, but I prefer losing such observations to the tremors and expense of a high structure.

The transit room has been used on several occasions by the U. S. Coast Survey in their telegraphic operations for longitude. It is Oh. 12m. 15-47s E. of Washington, and in latitude 40°, 43', 48"-53; the latitude being the result of observations with the zenith telescope upon twenty-four pairs of stars by the observers of the Coast Survey.

During the winter of 1857-58, Messrs. Alvan Clark & Sons constructed, and in the spring attached to the equatorial, a driving clock of the highest merit. It has a remontoir escapement similar to that of Bond's spring governor.

Having seen with great interest the photographic experiments conducted at the observatory of Harvard College, I determined, as soon as the clock should be in working order, to prosecute the subject of celestial photography. After many experiments it was ascertained that the best photographic focus of the objective was about of an inch outside the visual focus. I continued making photographs of the moon and such stars as could be obtained, and although when compared with what had been done by others the results gave reason for satisfaction, yet in view of what was desirable and apparently attainable, astronomical photography with me was a failure. By reducing the aper

ture of the telescope to five inches for the full moon, I was enabled to produce negatives which would bear an enlargement to five inches or fifty diameters. An impression of a sixth magnitude star was never obtained, 7 Virginis, then 3" distant, was the closest pair the duplicity of which could be measured on the collodion plate. The ring of Saturn and the belts of Jupiter were plainly visible, but entirely unsatisfactory. An image of Jupiter could be obtained in from 5 to 10 seconds exposure, but the satellites failed to impress the plate in any length of time. This was due to the uncorrected condition of the objective which diffused the violet rays over large space, so that in the case of the planet each point of the picture was influenced not only by the ray due to that point, but by the stray beams from adjoining portions of the object, and thus nearly the whole actinic. force of the objective was gathered within the dimensions of the image. In the case of the satellite the lost rays were not replaced by the wanderers from any adjacent point.

During the summer of 1858 I combined my first stereograph of the moon, producing quite a satisfactory result with the low power of the stereoscope. I do not know when this was first done in England by Mr. De La Rue, but with me the idea was an original one.

My greatest success with an uncorrected objective was in the pictures of the sun taken with about one-fiftieth of a second exposure, with the aperture reduced to one inch. The negatives were four inches in diameter and exhibited the spots with reasonable sharpness, the manifest difference in light between the center and the edge, and under favorable circumstances the faculæ. Some of the negatives verify the observation of M. Dawes, that the faculæ are elevations.

In June, 1860, the sun's disk was remarkably rich in spots, and I combined the pictures of two days to produce a stereograph, but the result was a failure and did not give the impression of a sphere, but presented the appearance of a flat uniform disk spanned by a spherical net-work which seemed entirely detached from the disk. This is attributable to a want of sufficient detail on the surface of the sun.

During the year 1859 and for a long time I worked with combinations of lenses to be inserted in the tube between the objective and the plate with the view of correcting the photographie ray. This attempt succeeded well so far as the center of the field was concerned, but it was impossible to produce a good correction over a space equal to the area of the image of the moon, without using a corrector of inconvenient size.

In 1860 I prepared a telescope with camera and instantaneous apparatus mounted equatorially to send by the U. S. Coast Survey Expedition to Labrador for the observation of the eclipse.