44 THE POPULAR SCIENCE MONTHLY. 



The most interesting of these are the rain-drop impressions, particu- 

 larly 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 exist- 

 ence. 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 loco- 

 motion. Bipedal tracks are chiefly of man, birds, and occasionally of 

 the kangaroo-forms. So readily can these be distinguished that defini- 

 tions 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 bur- 

 rows 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 investi- 

 gations must add to their number and definiteness, and consequently 

 to the value of ichnological studies. 



Cuvier has finely described the definiteness and certainty with 



