The most interesting of these are the rain-drop impressions, particularly as they indicate the formation of the tracks of animals upon a surface not covered by water. Every rain-drop will leave a single round impression. They are preserved most perfectly when it barely sprinkles. In a heavy or long-continued shower so many impressions are made that they coalesce and leave no distinct trace of their existence. They might be said to resemble a chopped sea. None of the latter could be recognized upon the rock, even if they existed; but the sparsely-scattered impressions are abundantly, oftentimes elegantly, preserved.
Furthermore, when rain-drops are blown by the wind, they must fall upon the mud at an acute angle, greater or less in proportion to the force of the current. When a small stream of water is made to fall upon a hard, fiat surface, it will be deflected, rising at the same angle, thus giving origin to the philosophical statement that the angle of incidence is equal to the angle of reflection. This principle is only partially exemplified by the Triassic phenomena, as the rain-drop is simply elongated in the direction of the wind. But these features illustrate the force and direction of the wind and the amount of the rainfall, so that we see the weather-cock and the rain-gauge of these ancient times. We find the fact impressed upon the strata in the same locality of a change in the course of the wind, showers, and storms, with, of course, intervals of sunshiny weather. Surely, then, the primitive times witnessed the same alternations of storm and sunshine that prevail at present.
The technical department of the science of Ichnology relates to a discussion of the characters derived from locomotion peculiar to each division of the animal kingdom. Certain distinctions are very obvious, such as the peculiarities of bipedal, quadrupedal, and multipedal locomotion. Bipedal tracks are chiefly of man, birds, and occasionally of the kangaroo-forms. So readily can these be distinguished that definitions are superfluous. The quadrupeds display hand-like feet, as the monkeys, rounded toes and heels like the dog, hoofs either single or cloven, and long, slender toes, few or numerous. Others, like the turtle and lizards, would show two rows of impressions, with short or long steps, and an occasional caudal mark. The lower forms of life would display a great variety of trails, loops, and hops; raised burrows in the mud, or vertical holes, and others of endless diversity. It is unnecessary to specify further the various locomotive characters by which the different groups may be recognized. That such exist may be considered as proved—some of them of very precise application. Not less than thirty different locomotive characters are made use of in the description of the New England ichnitic fauna. Further investigations must add to their number and definiteness, and consequently to the value of ichnological studies.
Cuvier has finely described the definiteness and certainty with
