4 GEOLOGY 



its history. A little heat and light are derived from other bodies, 

 and an important source of energy is found in the interior of the 

 earth itself; yet all of these are so far subordinate to the great flood 

 of energy which comes from the sun that they are quite inconse- 

 quential. The dependence of the earth on the sun has been intimate 

 throughout its past history, 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 in the province 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 like 

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

 with such velocities as to show that they do not belong to the sun's 

 family. Some consist almost wholly of metal, chiefly iron alloyed 

 with a little nickel (holosiderites) ; some consist of metal and rock 

 intimately mixed (syssiderites and sporadosiderites) ; and some 

 consist wholly of rock (asiderites) . It is now thought that the 

 meteorites throw some light, perhaps important light, on the early 

 history of the earth. They are therefore of interest to the geologist. 

 The amount of material added to the earth by their infall is now 

 relatively slight compared with the whole body of the earth; but 

 their contributions in the distant past may perhaps have been 

 greater. 



THE GRAND DIVISIONS OF THE EARTH 



The constitution of the earth. The materials of the earth fall 

 into three grand divisions: (1) The atmosphere, (2) the hydrosphere 

 (water sphere), and (3) the lithosphere (rock sphere). 



1. The Atmosphere 



Though the study of the atmosphere constitutes the science of 

 meteorology, the atmosphere is a part of the earth, and as a part 

 of the earth, it falls within the province of geology. It is an intimate 

 mixture of (1) all those substances that cannot take a liquid or solid 

 state at the temperatures and pressures which prevail at the earth's 



