tion, i. e., solidification of the magma, and any such process is, of course, impossible in a solid mass of rock.

This difficulty, however, does not seem to be insuperable. The evolution of heat on the solidification of molten lava is a well-known phenomena, having been actually observed, and the same has been experimentally verified, and the amount of heat evolved on the solidification of diabase has been determined by Barus.' Therefore the crystallization of minerals from a molten magma is an exothermic change.

It is therefore conceivable that the solidification of a laccolithic mass may give rise to sufficient heat to remelt portions of it, which might easily remain liquid long enough for a secondary differentiation to take place. That this was actually the case at Magnet Cove cannot be definitely shown, but it is at least an explanation which has a certain degree of probability in its favor.

In regard to problems of interpolation and extrapolation, by which Pirsson obtained such close agreement between calculated and observed chemical composition, we are not in a favorable position, since the analyses which I have made exhaust the known main types of abyssal rocks at the locality. A comparison of the analyses of the dike rocks given by Williams with the curves in Diagram 3 confirms the supposition expressed in my former paper that the tinguaites and the nephelite-porphyry are aschistic, and that the fourchites and ouachitites like the covite, are diaschistic.

It was expected that the analyses of the Fourche Mountain pulaskites would fall in with the curves as laid down. But, determining the abscissal position by silica, it is found that Al,O,, Na, O, and K,O are below, and (FeO), MgO and CaO are above, what would be their "normal" positions, as will be seen on reference to the diagram.

Although these rocks are unquestionably derived from the same general magma, yet, as their distance from Magnet Cove is about forty miles, it is clear that we need not be surprised to 'C. BARUS, Am. Jour. Sci., Vol. XLIII, p. 56, 1892.

find discrepancies, due to differences in the process of differentiation.

At Magnet Cove we have, without any doubt, the results of the differentiation in place of a small body of magma. This particular mass may have had originally the composition of the magma underlying the whole region, or it may have been itself a differentiate of this. Differentiation in such a large body of magma as that underlying the whole igneous region of Arkansas would naturally be likely to give rise to diverse products at different points, in which, however, could still be traced some of the original general characters of the whole. We are as yet scarcely in the position to deal with such intricate and obscure problems, but the results of Pirsson's investigations and of those embodied in the preceding pages seem to furnish a promising means of attack.

HENRY S. WASHINGTON.

PECULIAR EFFECTS DUE TO A LIGHTNING DISCHARGE ON LAKE CHAMPLAIN IN AUGUST 1900.

AFTER a period of long continued drought, when the ground. was very dry, a thunder shower arose in the Adirondacks, which passed east, crossing Lake Champlain over Westport and along south of the crest of Split Rock Mountain. No rain fell north of the top of the mountain, but a very severe storm passed to the south. When the storm had nearly disappeared, a sudden discharge of lightning passed down from the clouds, striking about half way down the northern slope of the mountain, entirely outside of the rain area and into the dry trees and rocks. In about a half a minute a cloud of what appeared to be dust could be seen rising from among the pines and juniper bushes. This, however, in a couple of minutes proved to be smoke, and in less than five minutes a very well developed forest fire was under way. Fortunately, a number of persons saw the discharge and saw the fire start, and immediately hastening to the spot were able to extinguish the fire before it had burned over more than a small fraction of an acre.

The peculiarity of the discharge was immediately observed upon coming upon the locality. An old pine tree seemed to have received the most severe part of the discharge and was badly split in the familiar manner. In addition to this, however, a number of places were immediately noticed where the lightning had struck either into the rocks or into the dirt overlying the rock. In two cases the discharge into the rock was of such force as to split the rock, tearing up fragments weighing as much as fifty to one hundred pounds and scattering them about. In other places the discharge upon the rock was comparatively slight, producing simply small fractures in the rock, and in some cases the effect was so slight as to simply remove the dry moss, leaving a small white spot not as large as the finger-nail. These partial

discharges of such varying force were scattered over an area of perhaps thirty to forty feet square, the more violent ones being within twelve or fifteen feet of each other. Upon examining the point at which any one of these discharges struck, a white incrustation was apparent upon the rock, as if white paint had either been spattered about or had been spread over as a rough, branching, straggling line. These white incrustations, in some cases, could be traced for a foot or more down into the cracks between the rocks. In other cases, they were mere spots. These white streaks were, undoubtedly, the paths along which the electricity ran, and a superficial examination showed that the white was due to an incipient fusion of the surface of the rock. Unfortunately, it was not practicable to get satisfactory photographs of these markings or to bring in large specimens. Small specimens, however, were brought in, and have been subjected to investigation.

The probable explanation of the scattering discharge of this particular lightning is to be found in the extreme dryness of the ground. The cloud charged with electricity would, of course, induce the opposite kind in the trees and rocks immediately beneath it. Then, when the discharge came, it was necessary that each prominence should discharge to the cloud individually, because the ground connecting the different prominences was too poor a conductor to rapidly collect the quantities of electricity and discharge them through a single point, as is usually the case.

COLUMBIA UNIVERSITY,

New York City.

WILLIAM HALLOCK.

A STUDY OF THE STRUCTURE OF FULGURITES.

SOME obscure problems in structures occurring within artificial and natural glasses (rhyolites, tachylites, etc.) receive light from the study of fulgurites, as representatives of instantaneous fusion, and frequently, as it will be shown, of equally rapid devitrification.

Hitherto, however, a constitution of homogeneous glass has been universally observed, under the microscope, in all sandfulgurites, without the least trace of devitrification in the largest masses.1 Occasional cloudy stains of brown iron-oxide and black manganese-oxide have been noted, and frequent enclosure of remnants of quartz-grains and of bubbles, both more abundant near the outer walls of fulgurite. In rock-fulgurites, a single instance of devitrification has been recorded. The results of examination of four fulgurites will now be described.

I. Fulgurite (lightning-tube) from sand, Poland; a small fragment, together with thin cross-sections, prepared by Mr. James Walker, of the New York Microscopical Society. This fulgurite is of small size, from 5 to 8 mm in diameter, with central aperture or lumen usually 2.5 to 4mm in diameter, and glass wall varying mostly from 0.6 to 2.0mm in thickness, roughened outwardly by adhering sand-grains in a continuous coating. The photomicrographs (Figs. 1 and 2) may serve to explain certain features as yet unrecognized in other fulgurites.

The wall presents, under low magnifying power, an apparently homogeneous glass, streaked by occasional cloudy wisps "A Perfect Glass," ARAGO (Ann. d. Ch. et. d. Phys., Vol. XIX (1821), p. 290) and all later investigators.

VON GÜMBEL, Zeits. d. D. geol. Ges., Vol. XXXIV (1882), pp. 647–648.

J. S. DILLER, Am. Jour. Sci., Vol. XXVIII (1884), pp. 252-258.

G. P. MERRILL, Proc. U. S. Nat. Mus. (1886), p. 84.

3 Best shown in the longitudinal and cross-sections of a sand-fulgurite by A.

WICHMANN (Zeits. d. D. geol. Ges., Vol. XXXV (1883), Pl. XXVIII).