116 NEW YOKK STATE MUSEUM 



ers of different densities, the rarer substances to some extent 

 occupying the outer portion of the mass. 



If, as this hypothesis suggests, our earth is an integral part of 

 the solar system we should expect to find its component elements 

 in the sun and in the other heavenly bodies, and this expectation 

 is confirmed by two distinct sources of information. Chemical 

 analysis of the meteorites which fall to earth shows that these 

 bodies contain many minerals which occur in the earth's crust a 

 and that they do not contain any elements which are unknown 

 on earth. Of late the application of the spectroscope to the 

 study of the sun and stars has established the fact that these 

 celestial bodies are largely composed of the elements already 

 known on earth. There are however some lines in the solar 

 and stellar spectra which are not matched by the lines in any 

 terrestrial spectrum. 



The conclusion to which we are led hj the nebular hypothesis, 

 viz. : that the earth originated as a rotating mass of incandescent 

 gas, is corroborated by its present form, which is that of a 

 spheroid of rotation or of a plastic body which, by rotation, has 

 become flattened at the poles. The difference between the polar 

 and equatorial diameters of the earth is about 27 miles. 



Chemical science has established the fact that all forms of 

 matter are composed of one or more of the elementary substances 

 or elements, of which there are 74. These are all found either 

 in the earth's crust, or in its atmosphere; they also occur in the 

 sun, stars and other heavenly bodies. Most of these elements are 

 very rare and do not come to the notice of the geologist. Only 

 11 are important as constituents of the earth's crust. These more 

 common elements are given in the following table 6 with their 

 proportionate percentages as components of the earth's crust: 



Oxygen 50 



Silicon 25 



Aluminum 10 



Calcium 4.5 



Magnesium 3.5 



Sodium 2 



a The crust is the superficial portion of the earth. & Prestwich Geology, p. 10. 



