LESSONS IN GEOLOGY. 310 



rapidly. They are all supposed to be in motion in definite direc- 

 tions but whether they move together as one system or as many 

 distinct systems is not known. 



The Nebulae are cloud-like masses, similar to the milky way, 

 which occur abundantly in different parts of the heavens. Many 

 of these, when examined with a telescope, are shown to consist of 

 distinct stars, others still appearing as clouds when viewed with 

 the most powerful telescopes. Some of these, when examined with 

 the spectroscope are shown to be star clusters, while others are 

 shown to be incandescent gaseous bodies. They differ in size and 

 in distance from the earth ; some are formless masses, some ring- 

 shaped and some spiral, as if the mass were beginning to rotate, 

 but our knowledge of them is very meager. The constitution 

 and conditions of the nebulae and stars ; the motions, conditions 

 and relations of the members of the solar system have given 

 rise to the Nebular Theory, which supposes that the sun was once 

 a mass of burning gases, which filled the orbit of Neptune. The 

 mass had a rotary motion which increased as it cooled and con- 

 tracted, becoming at length so rapid that portions were thrown 

 off, much as water and mud are thrown from rapidly turning 

 carriage wheels, this process continuing until all the planets were 

 formed, each assuming a spherical form, each revolving around 

 the parent mass, some and probably all rotating in the same 

 direction. Some of the planets rotated so rapidly that bodies 

 were thrown off from them and thus the satellites were formed. 

 This theory cannot be demonstrated but it harmonizes with most 

 of the facts known of the solar system. 



The nebulae as burning gas, the sun and stars as molten 

 masses surrounded by intensely heated gases, Jupiter and Sat- 

 urn as molten bodies which have cooled below the point of incan- 

 descence, the Earth and Mars as bodies, over which at least a 

 solid crust has been formed, are each supposed to represent dif- 

 ferent stages through which the earth has passed in developing 

 from the primal fire mist, to its present condition. The moon 

 probably represents a farther stage toward which the earth is 

 approaching. 



