PINE LAKE, WAUKESHA COUNTY, WISCONSIN.

These observations were made with the self-registering thermometer, Fahrenheit scale, manufactured by Hicks, of London; checked in same cases by Green's U. S. Signal Service thermometer.

A DESCRIPTION OF SOME FOSSIL TRACKS FROM THE POTSDAM SANDSTONE.

BY PROF. JAMES E. TODD.1

At the winter meeting of the Academy in 1879, a verbal description and discussion of these tracks, illustrated by photographs, was presented by Prof. T. C. Chamberlin, but the pressure of other work preventing the preparation of a description for the press, the matter was placed in the hands of the writer. The names here adopted are those then proposed. The specimens on which the descriptions are based in all about half a ton of slabs in the cabinet of Beloit College, and were procured through the kindness of Mr. Young and at the expense of Mr. Chamberlin.

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Several months since, Rev. A. A. Young of New Lisbon, Wis., called the attention of the state geologist to some very interesting fossil tracks, that occur at two quarries located near the Lemonweir river. They are about four miles north of the village of New Lisbon. The geological horizon is the upper portion of the Potsdam. The rock upon which they are impressed is a mediumgrained, compact, hard, silicious sandstone, which splits readily into flags, three or four inches in thickness. The conditions of its deposition are indicated by distinct, and often oblique lamination, and by ripple marks. No animal remains have yet been found associated with the tracks, though these are remarkably well preserved.

1. The general appearance of the tracks is of broad serpentine bands crossing the stone, and sometimes so thickly as to obscure one another, and give the appearance of an irregularly rippled surface.

The margins of the tracks appear to have been originally unbroken lines, and parallel. The whole surface between these lines has evidently been in contact with the animal making the track, and there are no signs that any part of the animal reached beyond these lines.

2. The most conspicuous element of the track consists of a closely consecutive series of nearly parallel, transverse ridges,

which, moreover, are not usually straight and exactly transverse, but most frequently V-shaped with the apex of the V pointing forward. This form, though the prevailing one, is nevertheless subject to the following modifications: The angle varies in the

PLATE 1-FOSSIL TRACKS ON POTSDAM SANDSTONE.
(From photograph.)

typical cases from the extreme limit of 110° to 135°. When the V becomes distorted, this angle appears to vary further, through larger angles, till, in various cases, it disappears, and the ridges are straight. This occurs somewhat more commonly in the smaller tracks.

Another frequent variation is in the relative length of the arms of the V, and the consequent shifting of the point towards one

side of the track. When the V remains at all regular this is toward the convex margin when the track is curved (vid. plate 1).

The surfaces of the ripple-like ridges are usually regularly convex, and this convexity is so nearly equal to the concavity of the furrows between, that from this feature alone, it would be impossible to say whether, in a particular case, you were looking upon the track itself, or upon the cast of it, found on the upper slab. Not infrequently the ridge is narrower than the furrow. Sometimes the converse is true. If either slope of the ridge is more inclined than the other, it is usually the posterior one, unless it has been modified by longitudinal lines, when the converse may be true. With this character may be connected the fact that from the way in which the stone breaks in the ridges, there is additional evidence that the ridges in their formation were pressed and moved backward somewhat.

Not infrequently the arms of the V are not formed together; sometimes they alternate for a time, sometimes an extra arm is intercalated between two V's, which are distorted, to adapt themselves to the case. Such cases occur more frequently in curves, but are not confined to them.

Sometimes the V-form of the ridge changes to a wavy line, a low W, a regular curve, or a straight line, and that within a short space, in the same track.

Sometimes the transverse ridges appear to fade out, as if the consistency was insufficient to sustain them after they were formed.

3. The third element of the tracks are the longitudinal lines. These are seen frequently modifying the tops of the ridges, aud forming the most reliable guide, in determining which impressions are the tracks, and which the casts; also, in which direction the animal moved. The track may also be sometimes distinguished from its cast, by its transverse section being a little concave or depressed.

These markings may be divided into three kinds.

First, those quite closely parallel with the sides of the track, as though formed by some appendages dragged over the ridges after they were formed. Of these, some seem to be made by rigid,