50 



POPULAR ILLUSTRATIONS OF 



THE EARTH'S CRUST. 



Azoic 

 (No life). 



Fundamental gneiss. 



* * * 



Granite. 



a 



2 



Total known thickness of the earth's crust containing remains of organic 

 life, 19,722 geographical miles. If the present estimate of Laurentian rocks 

 being 90,000 feet thick turns out correct, this amount will be increased to 

 31-062 miles. 



The estimate of the entire thickness of the earth's crust arrived at by Mr. 

 Hopkins, from data furnished by the effects of the sun and moon's attraction 

 as indicated by the rotation of the earth's axis and the precession of the 

 equinoxes, is from 800 to 1000 miles. 



