Beering's straits, including the great northern lakes and a considerable part of Hudson's bay. The isodynamic line of 1.85 is a smaller oval included within the former, and passing nearly through Fort Mackinaw. The isodynamic line of 1875 is an oval 446 geographical miles in length, and 170 in breadth. Its centre is in latitude 52° 19′ N., and longitude 92° w.; where the intensity is 1.878. These are the results which accord best with all the observations of Lieut. Lefroy and Prof. Locke. But it must not be understood that the observations agree perfectly among themselves. Many of the observations present unexplained anomalies, which, in our ignorance of their cause, we ascribe to local attraction. Thus while the great mass of observations assign to the magnetic focus above mentioned an intensity of 1-878, and indicate that from this point the intensity slowly diminishes in every direction, a few of the observations indicate an intensity considerably greater. In three instances, Lieut. Lefroy observed intensities greater than 2, viz., in lat. 48° 46', lon. 87° 40′, intensity 2.099; lat. 51° 44', lon. 96° 48′, intensity 2.031; and lat. 47° 37', lon. 85° 11', intensity 2.016. And in sixteen instances he observed intensities greater than 1878. Prof. Locke also observed intensities greater than 1878 in four instances, one of them rising as high as 1.950 in lat. 47° 28', lon. 88° 1. This point he inferred to be the point of maximum intensity for this continent; but Lefroy observed still higher intensities, and each of these cases was doubtless the effect of some local cause of limited extent. Within less than one mile's distance of the point where Prof. Locke observed the intensity 1.950, he found the intensity reduced to 1.842. The cause then, whatever it may be, of the high intensity on Porter's Island is extremely circumscribed in its influence. Prof. Locke has made some important observations to determine the laws of local attraction. At Patterson, N. J., on the summit of a trap rock one hundred and fifty feet high, he found the dip to be 750; while at the bottom of the rock the dip was only 72° 17'; making a difference of 2° 43′ within a distance of a few hundred yards. Near Fort Lee, about nine miles north of New York, on the ridge of the palisades, the magnetic intensity at two stations only forty feet apart, was found to vary by nearly one-twentieth of its whole value. Prof. Locke infers that trap rocks become magnets by terrestrial induction, diminishing the magnetism of the earth at their bases, and increasing it at their summits.

Lieut. Col. Sabine, in his magnetic survey of Scotland, mentions an instance of local attraction even more remarkable than the preceding. In the northwestern part of Scotland, near Lake Scavig, on a rock intersected by trap veins, he observed a dip of 78° 10. On the other side of the harbor, the dip was only 73°, being a difference of more than five degrees. These cases of local attraction deserve a particular examination.

ART. XVII. On the Trap Tuff, or Volcanic Grit of the Connecticut Valley, with the bearings of its history upon the age of the Trap Rock and Sandstone generally in that Valley; by Rev. EDWARD HITCHCOCK, President of Amherst College. (Read before the Association of American Geologists and Naturalists in Washington, May, 1844.)

In my Reports on the Geology of Massachusetts, I have given a brief account of a rock under the name of tufaceous conglomerate, connected with the sandstone and trap of the Connecticut valley, which appears to me to deserve more attention. For if I mistake not, we have in its history, a clue that will conduct to the solution of some difficult questions concerning the relative age of the trap and sandstone, and the period of the elevation of the latter. I have, therefore, given this rock a reëxamination in that part of the valley within the state of Massachusetts, and propose to present the result in this paper.

I understand all tufas of igneous origin, to have resulted from fragments of scoria and pumice, with dust from the same, falling upon the dry land, or into the sea, when thrown up by volcanic eruption. Sometimes they carry along with them fragments of other rocks, and when they fall upon the bottom of the sea they mix with the sand and gravel there, and it may be that melted matter from the same eruption mixes with the tufaceous matter, so that rocks of every grade are produced, from perfect trap to stratified sandstones and conglomerates containing a portion of volcanic matter and altered by heat. Such an origin and such a composition correspond with the rocks which I am about to describe. I have observed in them the following varieties.

Lithological Characters.

1. A hard reddish micaceous sandstone, with more or less of volcanic matter, mostly dust or scoriæ. It is more or less metamorphic, often abounds in greenish spots, and passes insensibly into pure sandstone.

2. Conglomerates.-There are several varieties. The most common consists of rounded masses of trap, and occasional masses of red and grey sandstone, imbedded in a scoriaceous base. The nodules vary in size from that of a pea to masses two feet in diameter. The trap nodules are chiefly a fine grained greenstone. They are usually more or less rounded, but do not appear like the smooth pebbles in conglomerates of an entirely aqueous origin. They have evidently been worn mechanically, but their surfaces have been acted upon by heat, so as to have lost their smoothness.

Other varieties take into their composition, in addition to the trap nodules, distinctly rounded pebbles of older rocks, such as

granite, quartz rock, clay slate, &c. These rocks appear more or less crystalline in the structure of their cement, and contain, especially, foliated nodules of calcareous spar.

3. Volcanic Breccia.-This is composed of angular fragments of trap, of a highly ferruginous character, with a small proportion of cement of the same character. This rock abounds in the large

ridges of intrusive trap in the vicinity.

4. Amygdaloid, with a somewhat compact base, and nodules of calcareous spar. The base has the aspect of indurated volcanic mud.

5. Volcanic Slags.-These differ but little from the amygdaloid, except in being more vesicular, and the cavities are empty. They cannot be distinguished from recent vesicular lava, except in not being so fresh.

6. Concretionary Nodules.-These are made up of concentric coats and are a dirty color and highly ferruginous. They are rarely more than three or four inches in diameter.

7. Common Greenstone.-This differs not at all from the greenstone that constitutes the great mass of the trap of this valley, except perhaps in being usually more compact.

8. Porphyritic Trap.-This takes distinct crystals of feldspar into its composition, and has a gray base more argillaceous than greenstone. The same rock occurs in connection with the principal ranges of trap.

Relative Position and Stratification of these Deposits.

The usual dip of the strata of sandstone in the valley of Connecticut, is easterly, averaging about 15 or 20°; and the large ranges of trap appear to have been protruded to the surface between the strata. Consequently they lie upon the sandstone as seen on the west side of the range; while on the east side, the sandstone lies in an inclined position upon the trap, frequently with a higher inclination than the sandstone beneath. The east side of the trap, therefore, I call the upper side, and the west side, the under side. Now it is upon the upper side only that the trap tuff occurs. After receding a greater or less distance from the trap range, we find the tuff interstratified with the sandstone, as shown on the sections, figs. 1 and 2; and the tuff is also, for the most part, distinctly stratified conformably to the

Fig. 2.

men when Sand Trap
Sandstone.
Trap. stone. tufa.

Section across Mount Holyoke southeasterly.

sandstone. I say for the most part; because when the mass is considerably thick, the west or the lower portion of it, is usually distinct greenstone, having no more stratification than trap in any other position. And so in the amygdaloidal and scoriaceous varieties, the stratification is indistinct, and indeed,. in proportion as the igneous agency predominated, the rock loses its division into layers, and exhibits it where the aqueous agency predominated.

It ought also to be stated, that the amygdaloidal, scoriaceous, and porphyritic varieties of this rock occur of considerable thickness on the upper side of the principal ranges of trap, and to some extent they are stratified. The brecciated variety is, also, common, capping and underlying the columns, in various parts of the range. I have never, however, found the tufaceous conglomerates and sandstones as a part of the main range of trap, unless it be in West Springfield: where I was not able to find any sandstone between the conglomerate and the main range of trap.

Topography.

Nearly all the examples of this rock which I have discovered, occur upon what I call the back or southeasterly side of Mounts Holyoke and Tom. These form an almost continuous ridge, extending from Belchertown in Massachusetts, nearly to New Haven in Connecticut. Starting at Belchertown; we find the ridge running nearly west for several miles, then turning south to Connecticut river, and forming Mount Holyoke. Continuing across the river, it rises into the still higher ridge called Mount Tom; and thence in a lower ridge, into Connecticut. The top of this whole ridge is common trap, prismatic, amygdaloidal, and scoriaceous, resting upon thick bedded sandstone, which crops out upon its western and northern side; and on its other side, grey and red micaceous sandstone and shale shoot up upon its back. Now it is within this great curve of the principal trap range, and on its back side, that all the deposits of trap tuff, with which I am acquainted, occur. Three of these are shown on the map, Plate I, on the south side of Holyoke, in Granby and South Hadley. There may be others there: but the country being chiefly covered with thick woods, it is difficult to explore it. On the opposite side of Connecticut river, a still larger deposit occurs in Northampton, SECOND SERIES, Vol. IV, No. 11.-Sept., 1847.

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on the back side of Mount Tom. In West Springfield, another narrow deposit is marked, as if imposed directly upon the principal range of trap; though it is doubtful whether there may not be sandstone interposed. Some very interesting boulders of one variety of this rock have been found in Anıherst, torn off probably from the back side of the trap range running northerly through Sunderland, Deerfield, and Greenfield, where we still find a similar rock forming a part of the range. I have, also, seen many years ago, boulders of the tufaceous conglomerate, a few miles south of Hartford; but whether any large ledges exist there I know not.

Organic Remains.

The extreme paucity of organic remains in the new red sandstone of New England, and the highly igneous character of most varieties of the trap tufa, would lead us scarcely to expect any animal or vegetable relic in the rock under consideration. But in fact a single very distinct example does occur. It is a vegetable stem, from one to three inches in diameter, scarcely flattened. I found it in boulders of the porphyritic trap that has been described, scattered in abundance over the fields, in Amherst. I have not been able to trace it to its place in the mountains. But a similar rock abounds along the east side of the trap range running through Deerfield and Greenfield, and that too in just the direction from which the drift of Amherst has been derived. But in

the ledges I have not found any vegetable stems. The largest boulder in which I have found these stems, is about two feet in diameter, and three of these stems run parallel to one another entirely through it, and do not diminish much in size, so that probably they were five times that length. The rock is a dark gray trap, not at all vesicular, yet occasionally containing prehnite and being porphyritic. But the stems are changed into what I have called a volcanic slag, being highly vesicular, resembling the toadstone of the geologists. I think this fact may be explained very reasonably; but it will be better understood when I have presented the theory of the formation of these peculiar

rocks.

Still

I ought to add, that I have been able to procure several of these unique specimens; but the best and largest has been divided between my own and the state collection of Massachusetts. farther I should state, that I have given a description of them with a poor drawing, in my Final Report on the Geology of Massachusetts; although I was there obliged to refer them to real trap rock, not having understood, as I think I now do, the relations of the trap tufa.