4 GEOLOGY. 



for Titan, probably the largest, is only ^o o of the mass of Saturn. There 

 is little doubt that the moon has played an important part in the history 

 of the earth. It is the chief agency in developing oceanic tides, and 

 it possibly also develops a body tide in the earth itself. These tides 

 act as a brake on the rotation of the earth and tend to reduce its rate, 

 and thereby to lengthen the day. While this may h^ ^^ been counter- 

 acted in some measure by the shrinkage of the earth ^hich tends to 

 increase its rate of rotation, it has been held by eminent physicists 

 and geologists that the rotation of the earth has been greatly lessened 

 during its history, and that a long train of important consequences 

 has resulted. If the contraction of the earth has been sufficient to 

 offset this lessening, the tidal brake must be credited with the preven- 

 tion of the excessive speed of rotation which would otherwise have been 

 developed. The tides are efficient agencies in the shore wear of the 

 oceans, and in the distribution of marine sediments, and these, it will 

 be seen later, are important elements in the formation of strata. 



Dependence on the sun. — By far the most important external 

 relation of the earth, however, is its dependence on the sun. The earth 

 is a mere satellite of the' Sun, less than ^o^ofo of its mass, and hence 

 under its full gravitative control. The earth is dependent on the sun 

 for nearly all its heat and hght, and, through these, for nearly all of the 

 activities that have given character to its history. It is too much 

 to say that all activities on the surface of the earth are solely depend- 

 ent on those of the sun, for a certain measure of heat and hght and 

 other energy is derived from other bodies, and a certain not inconsid- 

 erable source of energy is found in the interior of the earth itself; yet 

 all of these are so far subordinate to that great flood of energy which 

 comes from the sun that they are quite inconsequential. The history 

 of the earth in the past has been intimately dependent upon that of 

 the sun, and its future is locked up with the destiny of that great 

 luminary. Geology in its broadest phases can therefore scarcely be 

 separated from the study of the sun, but this falls within the func- 

 tion of the astronomer rather than the geologist. 



Meteorites. — There are a multitude of small bodies passing through 

 space in varying directions and with varying velocities and occasionally 

 encountering the earth, to which they add their substance. Some of 

 these meteorites revolve about the sun much as if they were minute 

 planets, but some of them come from such directions and with such 

 velocities as to show that they do not belong to the solar family. Some 



