foot of the mighty Brontozoum. On the reversed surface of the same slab are found impressions, which were produced by a number of fragments of sticks, five or six inches long, lying at right angles, or nearly so. One of these sticks has been broken, and its pieces are slightly displaced from each other. Various other specimens contain the marks of sticks, or twigs of trees. The striæ, so distinctly discernable in a number of these portions, having been compared with twigs of the existing coniferæ (?), were found to resemble them. Some of these sticks show the appearance of incipient carbonization; yet the rock is sandstone, presenting, as already mentioned, distinct appearances of quartz, and other substances of which the arenaceous rocks are composed.

PHYSICAL IMPRESSIONS.

The third great division of impressions in the sandstone rocks is called Physical, meaning those made by inanimate and unorganized substances; such are rain-drops, ripple-marks, and coprolites.

1. Marks of rain-drops, described on page 20, appear to be quite common. We have two or three specimens in relief, and as many in depression. They occur as follows: 1st, on the upper surface of the slab first described; 2d, on that of the Platypterna; 3d, on that of the Æthyopus Lyellianus; 4th, on that of the Brontozoum Gracillimum; 5th, on that of the Æthyopus Minor; 6th, on that of the Anomœpus Scambus; 7th, on the recent clay; also in one small hand-specimen, and in a second containing two fishes. They show that, in those ancient periods when the Brontozoum Giganteum and the Otozoum resided in these parts, showers were frequent, and probably abundant for the supply of the wants and the gratification of the appetites of these animals, then common, but which now appear to us so extraordinary.

2. Ripple-marks are seen in a number of these pieces; for example, on the slab first described, on the Brontozoum Sillimanium slab, on the Brontozoum Gracillimum slab, on one of the Triænopus, and on the upper surface of the Greenfield slab. These marks are represented by parallel curves, or straight lines, distant from each other from half an inch to an inch, and presenting a slight degree of prominence. There is another form of ripple-marks(?), differing from those above described. These are of a circular and mammillary form: they are strewed thickly, like little islets, approximating to each other. They are seen distinctly on one of the slabs of the Brontozoum Sillimanium, on that of the Æthyopus Lyellianus, and some others. Whether they are to be considered as accumulations of sand and clay, formed by the action of the sea, we are uncertain; but there seems to be no other cause to which they can be assigned with so great probability.

3. Coprolites, the fossilized ejections of animals, are intermixed with other animal vestiges in the sandstone of Connecticut River, and afford additional proof of the former existence of animals about these rocks.

The latest accounts of fossil footprints we have had occasion to notice are those of the Crustacea, already mentioned, as found in Canada, and of the Chelonian in Scotland. The Canadian impressions, called by Professor Owen Protichnites, were discovered in the year 1847, and were laid before the London Geological Society in 1851. The most remarkable circumstance about them was their existence, as already stated, in a white sandstone, near the banks of the River St. Lawrence, at Beauharnais. This sandstone, which has been described by New York geologists under the name of Potsdam, is thought to belong to the Silurian system, and to have a higher antiquity than even the "old red."

The Scotch footsteps are situated in the old red sandstone, and are those of a Chelonian. So that we have now two series of tracks, the Crustacea in Canada and the Chelonian in Scotland, of higher antiquity than any which had been previously discovered.

On a review of the labors of President Hitchcock, we are struck with admiration at the immense details that, in the midst of arduous official and literary duties, he has been able to go through with in the period since the foot-tracks were discovered on Connecticut River. Although his labors should be modified by succeeding observers, Science must be ever grateful to him for laying the foundation, and doing so much for the completion, of a work so great, novel, and interesting.

This inquiry seems to us to promise a rich variety; and we hope that President Hitchcock and other observers will continue to explore and cultivate it with undiminished zeal.

DESCRIPTION OF THE PLATE.

We are indebted to Photography for enabling us to represent the remarkable slab from Greenfield, and its numerous objects, in a small space, yet with perfect accuracy. This slab is four feet seven and one-half inches in one direction, and four feet one inch transversely to this; in thickness it measures about an inch. It is composed of gray sandstone, in which the micaceous element is conspicuous, and contains many interesting impressions on both surfaces.

The most interesting surface is the inferior; and the objects are, of course, presented in relief. They are, first, two Chelonian tracks; second, four sets of bird-tracks; third, footsteps of an unknown animal. The Chelonian tracks are two in number: the longest measures four feet ten inches; the shorter, two feet nine inches. Both of these impressions are made apparently by the plastron of the turtle. They are from four to eight inches in width, and composed of elevated striæ. These striæ are formed by raised lines, pursuing a course generally regular, but accompanied with some inflections: they are, as the plate represents, very distinct. The shorter track appeared to me to be crossed by another; but the photographic impression, though only a few inches long, enabled me to ascertain that this appearance was produced by bird-tracks above and below.

The bird-tracks are all tridactylous. The first set lies above and to the right of the shorter turtle-track, and is composed of only two steps, proceeding in the course of the plate downwards. The second set of bird-tracks has five impressions, extending from the right superior pointed angle of the slab across the small turtle-track to the larger, in which it is lost. The third set of bird-tracks begins by an impression larger than any other on the piece at the left extremity of the longer turtle-track; and the remainder, three in number, descending towards the right, are the least distinct of any. The fourth set of bird-tracks begins below the longer turtle-track, and ascends by four impressions, crossing the track till it meets the first.

The most curious track, consisting of six digitated impressions, still remains. The first is seen on the left of the longer turtle-track, near the largest bird-track; the second is on the track; the third is above the track; the others cross the slab by fainter impressions. Each of them is composed by two feet, and each foot contains four toes, which are seen more distinctly in some impressions than in others. The largest of these double tracks is about three inches in diameter. Perhaps it would be useless to speculate upon what kind of animal they were made by. There is a similarity between these and the tracks of the Anomœpus Scambus, spoken of