The rock is fine grained, of a light yellow color and very friable. Some of the superior beds, which are thin, have been wrought for grindstones. The friable character of this sandstone is one of its most prominent features, and, owing to this circumstance, the escarpments are not usually high, or abrupt, unless it has been protected by the overlying rock. In its want of cohesion, it differs, in a very marked degree, from the prevailing character of this rock, as developed in New York and Canada, where it is usually, though not always, compact. It is not, however, unlike the sandstone of the Pictured Rocks, and is less friable than that of the Mississippi and St. Croix region.

The almost uninterrupted continuity with which this rock can be traced, even from its eastern extension through Canada and along the northern shore of Lake Huron to the St. Mary's river, and thence westerly, leaves no doubt as to its true position and identity in age with the Potsdam sandstone of New York. If we were at a loss in thus tracing it continuously, we have still the evidence of the succeeding fossiliferous strata, which show, conclusively, the same relations to this sandstone as they do to its equivalent in New York. With both these evidences combined, we cannot hesitate for a moment in our conclusion regarding its age and place in the series.

From the points just noticed, where this sandstone appears in eastern Wisconsin, it can be traced uninterruptedly across the entire breadth of the state to the Mississippi and St. Croix rivers. It is true, at the last named localities, we have the evidence of fossils which are not known to occur in its easterly extension; but we have already noticed the occurrence of the Trilobite on the Menomonee, while we have the Lingula everywhere, though in far greater profusion in the St. Croix region than elsewhere. In drawing inferences as to the age of the rock, from the occurrence of these fossils, it should be remembered that it is by no means improbable that similar ones may yet be found in more easterly localities. They seem to be coëxistent with calcareous bands, or the more calcareous portions of the group, and it is to this modification that we should look for the development of the fauna of this ancient period.

From all this evidence, we regard the question of the age of this rock as settled-that the Potsdam sandstone of New York is identical with that of the Mississippi and the St. Croix. One great source of doubt and perplexity in its determination, heretofore, was the recurrence of a sandstone identical in character with the lower, but superior in position to the calciferous sandstone, or lower magnesian limestone. It is a thin mass, evidently due to a recurrence of the same causes which produced the inferior deposit. This has been well elucidated by Dr. Owen in his reports on the upper Mississippi, in which he has shown that,

near the junction of the lower sandstone with the calciferous, there are several alternations of calcareous and silicious bands, the latter having the character of the sandstones below, and the former of the calcareous deposits above. These occur in several places on the upper Mississippi river, and give the geologist an introduction to that condition of things which subsequently produced the upper sandstone, which is distributed over a large part of Wisconsin, so often mistaken for the lower member of the series; but which, in fact, is separated from it by two or three hundred feet of calcareous rocks.

This upper sandstone can be regarded in no other light than as the result of the same causes which produced the Potsdam, and were suspended during the period of the deposition of the calciferous sandstone, or lower magnesian limestone, to be renewed, for a short period, in the deposition of a mass of sandstone, varying from fifty to eighty feet in thickness, upon the surface of the calcareous deposit. This fact shows the more intimate connection between these two lower groups than has heretofore been suspected. It is, nevertheless, shown in many places within the Lake Superior district, that the true sandstone, as it is traced upward, becomes gradually calcareous, and "finally passes into well-characterized, compact, magnesian limestone."* The same is true, also, of this rock, in Canada and New York; while, however, there is rarely any evidence of increase in the silicious materials towards the termination, as we observe in the west. In some localities, there are thin but distinct bands, near the upper portion, having an oolitic structure, which, as we go westward, appear to be replaced by beds of a granular texture and of a silicious character.

ART. III.-Analysis of Tin Pyrites; by Dr. J. W. MALLET.†

THIS rather rare mineral is one of which the chemical composition has appeared somewhat doubtful, owing to the considerable discrepancy between the three or four analyses which have been made of it, and to the fact that it almost invariably occurs massive and so intimately mixed with copper pyrites and other minerals as to render it difficult to select a fair specimen for examination. The locality from which most of the specimens in cabinets have been derived is Wheal Rock, near St. Agnes, Cornwall; but the mineral has also been observed in two or three other Cornish localities, and at Zinnwald, in Bohemia. I have recently received a specimen (name unknown) from a friend in England, which he states to have been found on St. Michael's Mount, + Communicated for this Journal.

Part 1, p. 117.

SECOND SERIES, Vol. XVII, No. 49.- Jan. 1854.

5

Cornwall, and which, on examination, proves to be tin pyrites, and apparently in a purer state than any hitherto analyzed.

This specimen occurs in quartz which has obviously been taken from a vein in granite. The structure appears to be crystalline, although no distinct planes could be observed. The color is not steel-gray, as in that of the mineral from Wheal Rock, but iron-black, with slight superficial blue and red tarnish in some places. Streak black, lustre sub-metallic, fracture uneven. Hardness 4. Sp. gr. =4.522. Heated before the blowpipe, on charcoal, sulphurous acid is given off, oxyd of tin deposited in large quantity upon the charcoal, and a black globule obtained, from which copper and tin may be reduced on the addition of soda.

=

A carefully conducted quantitative analysis, in which chlorine was used to decompose the mineral, gave the following results.

Thus the relative number of atoms of sulphur, tin, copper, and iron and zinc, as given in the 2nd and 3rd columns above, are almost exactly as 8: 2:4:2; whence we have the formula first assigned by Kudernatsch, (Pogg. Ann., xxxix, 146.) viz., 2(Fe S+ZnS), SnS2+2Cu2 S, SuS2. The present analysis agrees so closely with this formula, from which the results of Kudernatsch, and even those of Rammelsberg, (Handw. d. Chem. Theils d. Mineral. 2d Suppl., 179,) sensibly differ, that it seems fairly to be considered as representing the composition of the mineral in a pure state. The analysis also possesses some interest in showing the presence of zinc in considerable quantity, therein agreeing with Rammelsberg's analysis above referred to of the mineral from Zinnwald. It is to be observed that in both cases the iron and zinc occur in very nearly atomic proportions, so that perhaps the formula should be written 2FeS, SnS: +2ZuS, SUS: +2(2 Cu S, SnS), though this does not seem very probable, since Kudernatsch found 12:44 p. ct. of iron to but 177 of zinc, while Johnston gives 10.113 p. c. of the latter to 4791 of the former, as contained in tin pyrites (from the same locality as the specimen submitted to the present analysis, St. Michael's Mount). The presence of this large quantity of zinc, is however of importance, chiefly as proving that the tin must enter into the composition of the mineral as bi-sulphuret, since the other formula which has been proposed, namely, 2SnS, FeS2 +2Cu2S, FeS2, would

require us to admit the presence of ZnS2, whereas no such compound is yet known either occurring in nature or formed by artificial means. It is remarkable that Fahlerz, the only other compound sulphuret in which zinc occurs in notable quantity, is the mineral whose composition (4(RS), SbS3) seems to approach most nearly to that of tin pyrites, the latter containing two instead of four atoms of the sulphur bases to one of sulphur acid, and this electro-negative sulphuret being bi-sulphuret of tin, instead of ter-sulphuret of antimony. Indeed, some real connection between these two minerals seems to be further indicated by their occurrence in the same crystalline system, and their close resemblance to each other in hardness, specific gravity, and general physical characters.

ART. IV.-Notice of the Hail Storm which passed over New York City, on the first of July, 1853; by ELIAS LOOMIS, Professor of Mathematics and Natural Philosophy, in the University of the City of New York.

On the first of July, 1853, a very remarkable hail storm passed over the city of New York. The day had been uncommonly hot and sultry, the thermometer having risen to 90 degrees, and the air was believed to contain an unusual amount of vapor. A little before 5 o'clock in the afternoon, a heavy black cloud was observed to rise in the northwest, the wind at the time blowing moderately from the northeast, and subsequently from the east. As the cloud advanced and covered the northwestern sky, while it was still clear in the southeast, numerous streaks of zigzag lightning appeared to issue from the front margin of the cloud and descend towards the earth. I noticed the approach of the storm from my lodgings in Eighth street, within a quarter of a mile of the University. About five o'clock the wind came strong from the northwest, and the rain poured down in torrents. ently I heard a loud thump upon the roof of the opposite house; soon another thump; and presently a third and fourth. I was not long in discovering that the noise was produced by the fall of hailstones of a size such as I had never before witnessed. They were few in number-but their average size was little less than that of a hen's egg; and one or two I am persuaded were fully as large as my fist. They almost invariably broke on striking the pavement; so that I could not secure either of those large stones except in fragments; and moreover the rain was falling in torrents. I however hastened to the yard in the rear of the house, hoping to find some upon the grass which had not been broken in the fall. After the rain had nearly subsided, we

Pres

found several handfuls of hailstones of good size, though altogether inferior to those which I saw in the street. They generally consisted of very transparent and solid ice, with many air bubbles; but they were not spongy in the centre as they are sometimes found. Hailstones sometimes occur which appear to be little more than pretty compact snow-balls. In the present instance, the hail was not of this kind. The large stones, however, generally consisted of an irregular assemblage of angular pieces of ice, which individually did not much exceed the size of hazel nuts but they were cemented very firmly together. Indeed there was no appearance of seams or joints between these individual portions-but the ice was equally strong throughout every part of the mass. Their structure therefore did not indicate that several small hailstones were seperately formed and were subsequently cemented together; but rather that all were formed simultaneously about a common nucleus. Several persons independently, and without concert, suggested that the conglomerated mass resembled rock-candy: and the comparison appeared to me to be a very just one. There was a decided appearance of a tendency to crystallization. This tendency was in many cases towards a pyramidal form; in others they bore a resemblance to hexagonal prisms; and in some it appeared to me the tendency was towards a cubical form-though as the angles were all much rounded by the melting of the ice, I did not find any complete cubes.

Several of the stones which we picked up in the yard measured two inches long, and one measured over two and a half inches. These had been lying several minutes in a warm drenching rain; and it is my full conviction that two or three of those which I saw in the street were three and a half inches long, by two and a half inches wide, and they did not appear to deviate much from the spheroidal figure. A friend of mine, who is by profession a painter, and who saw and handled the hailstones, at my request made a sketch of some of the most remarkble of those which we picked up. These are shown in the accompanying figures which are drawn of the natural size. to be understood however that these were unquestionably smaller than many which we saw fall.

It is

The rain, accompanied by thunder and lightning, continued for six or eight minutes, when its violence somewhat abated-it returned again with renewed energy, but soon afterwards entirely ceased. Another, but more moderate shower followed half an hour later, yet without either hail or lightning. Throughout the entire storm, the wind had blown with considerable force, but not with destructive violence, in that part of the city which is southwest of the University; and in the lower part of the city there was comparatively little wind.