COMPONENTS OF THE EARTH'S CKTTST, MINERALS AND ROCKS 119 



lines of mountain making and that probably the outflows of 

 molten rock are due to local relief of pressure by some upward 

 movement within the mountain masses. 



Envelopes of the Earth 



The earth, besides possessing a solid crust and an intensely 

 heated interior, has two fluid envelopes. 



The gaseous envelope or atmosphere, which consists of the air 

 we breathe, surrounds the entire globe. 



The liquid envelope, of which the various portions are known 

 as oceans, seas, gulfs, bays, lakes, etc., envelops the globe only 

 in part, the exposed portions of dry land being known as islands 

 and continents. 



These two envelopes, under the influence of physical forces, are 

 very active agents of destruction, transportation and deposition 

 in their action on the earth's crust. 



The present relations of the envelopes to the continents, the 

 forms of the latter, the causes of climate, the origin of the winds 

 and ocean currents are usually discussed under the head of 

 physical geography. As this subject is not at present illustrated 

 in the State Museum, the student is referred to the many excel- 

 lent text-books on this science. 



COMPONENTS OF THE EARTH'S CRUST, MINERALS 



AND ROCKS 



The earth's crust consists of aggregates of matter which occur 

 in stratified and unstratified masses and are known as rocks. 

 The chemical combinations which form these rocks either singly 

 or in mixture are called minerals. The minerals, therefore, all 

 possess a definite chemical composition which can be expressed 

 by formulae. Books vary in composition, as they consist of one 

 or more minerals. The rocks which are mixtures of several min- 

 erals vary in composition as the proportions of their components 

 vary; and it is possible for specimens taken from the same rock 

 mass to differ in chemical composition. 



