IN THE RED SANDSTONE OP POTTSVILLE. 
11 
marks or grooves on as many ripple lines, the crests of which lines are elevated about 
half an inch above the intermediate depressions. The tail was evidently not a thick one, 
and the animal must have had a distinct and perfect step, and not a half swimming 
motion, as in the crocodilians, there being no trace of the dragging of the feet. The tail 
must have been considerably elevated, as the alternate tail-impressions show that a vibra¬ 
tion actually took place at every step, the four grooves not being in a direct line, but 
each one approaching its nearest “foot-mark” to the right or the left, alternately, and 
therefore never precisely on the central line between the two rows of the foot-marks. 
These facts prove that the animal which left its imprint in this ancient sandstone stood 
much higher on its legs than the Crocodilus or the Monitor , and likely was not so long in pro¬ 
portion to the size of the feet. It is well known that the alligator leaves no foot impres¬ 
sion in the mud, but simply a large furrow, made by the ventral and caudal portions. 
The form of the foot impressions are, however, very similar to that which is received by 
the mould in clay of the Alligator Mississippiensis , specimens of which are in the collection 
of the Academy of Natural Sciences of Philadelphia. If an opinion might be hazarded 
as regards the probable size of the animal, based on this meagre diagnosis, I should 
suppose it might reach as much as seven or eight feet in length. 
The “ ripple-marks,” as observed before, covered the whole surface of the exposed 
part of the rock, six by twelve feet. Their crest-lines are generally seven or eight inches 
apart, and they swell up gently in nearly parallel lines, but falling off and recommencing 
again, give an irregular appearance to the surface, but which is altogether harmonious. 
The “rain-drop pits” are rather large and nearly round, showing a tranquil state of 
the atmosphere when the rain descended. They covered the whole surface, and are not 
very deeply impressed. 
In regard to the geological position of the “ foot-marks” of this reptilian quadruped, 
having had no reason to doubt my former views, originally expressed in the Proceedings 
of this Society, I shall state it here nearly in the same words. The discovery was of 
great interest from the fact that no such animal remains had before been discovered so 
low in the series. Those described by Dr. King, in the great western coal-field, which 
naturally created great interest, are only eight hundred feet below the surface of the coal 
formation. (No. 13, of Prof. Rogers.) The position of the Pottsville “foot-marks” is 
about 8500 feet below the upper part of the coal formation there, which is 6750 feet thick, 
according to Professor Rogers, and they are in his “ Red Shale Formation,” (No. 11,) 2949 
feet thick, the intermediate silicious conglomerate (No. 12,) being stated by him to be 1031 
feet thick at Pottsville. These measurements would bring these foot-marks about seven 
hundred feet below the upper surface of this “ Red Shale Formation,” (No. 11,) which is the 
Old Red Sandstone of Mr. Taylor, Mr. Hall and other geologists of the New York Sur¬ 
vey,* and now known as the Devonian of Sedgwick and Murchison. It has been well 
* See R. C. Taylor “On the Carboniferous Series of the United States;” “ Report of the Dauphin and Sus¬ 
quehanna Coal Company;” “Geological Chart of the Silurian Rock Formations of North America,” MSS. 1847, 
in the Library of Acad, of Nat. Sci., &c., James Hall, “ Geology of New York,” Part 4, pp. 278, 516, &c., 
and other of these New York Reports. 
