LITHOLOGICAL STUDIES. 



CHAPTER L 



Sectiojj I. — The Structure of the Earth. 



The "natural system of rocks" ought to contain within itself a key to 

 the history of the earth, and to be an exponent of that history. The entire 

 physical universe seems to be under the government of a universal law, and 

 our classification ought to accord with that law, so far as it has been ex- 

 pressed in the rocks themselves. This law has thus far been best formulated 

 by Sir William Thomson as the degradation and dissipation of energy, — or, as 

 it may be styled, the passage from the unstable towards a more stable 

 condition, — the tendency towards harmony with the environment. This I 

 regard as the law under which the universe has moved from its beginning, 

 and under M'liich it will continue its course uniformly towards the end, 

 believing that no turning back can occur, and that no energy once lost 

 can be restored except by the same Almight}^ Power which gave it birth.* 

 It is in accordance with this law that I have tried to do my work, and to 

 set forth the principles on which the rocks described are classified ; in 

 other words, this is an attempt to explain the present condition of the rocks 

 by tracing out their past history. 



The terms " lithology," " petrography," and '• petrology " are so indefi- 

 nitely employed that it seems necessary to give some fixed meaning to them 

 here. For this purpose I define Lithotogg as that science which treats of 

 the constitution and pliysical structure of rocks. It corresponds somewhat 

 to the anatomy and histology of animals, including the study of morbid 

 tissues. 



* Science, 1SS3, i. 541. 

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