EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 



By Joseph Le Conte. 



INTRODUCTION — SOURCES OF ENERGY. 



Nearly all the processes of nature visible to us — well-nigh the whole 

 drama of nature enacted here on the surface of the earth — derive their 

 forces from the sun. Currents of air and water in their eternally 

 recurring cycles are a circulation driven by the sun. Plants derive 

 their forces directly, and those of animals indirectly through plants, 

 from it. All our machinery, whether wind driven, or water driven, or 

 steam driven, or electricity driven, and even all the phenomena of intel- 

 lectual, moral, and social activity have still this same source. There 

 is one, and but one, exception to this almost universal law, namely, 

 that class of phenomena which geologists group under the genernl head 

 of igneous agencies, comprising volcanoes, earthquakes, and more grad- 

 ual movements of the earth's crust. 



Thus, then, all geological agencies are primarily divided into two 

 groups. In the one group came atmospheric, aqueous, and organic 

 agencies, together with all other terrestrial phenomena which consti- 

 tute the material of science; in the other group, igneous agencies and 

 their phenomena alone. The forces in the one group are exterior ; in the 

 other, interior; in the one, sun derived; in the other, earth derived. 

 The one forms, the other sculptures, the earth's features; the one 

 roughhews, the other shapes. The general effect of the one is to 

 increase the inequalities of the earth's surface, the other to decrease 

 and finally to destroy them. The configuration of the earth's surface, 

 the distribution of land and water — in a word, all that constitutes 

 physical geography at any geological time — is determined by the state 

 of balance between these two eternally antagonistic forces. 



PHENOMENA TO BE STUDIED. 



Now the phenomena of the first group, lying, as they do, on the sur- 

 face and subject to direct observation, are comparatively well under - 



1 Annual address by the president, Joseph Le Conte, read before the Geological 

 Society of America, December 29, 1896. Printed in Science, Vol. V, No. 113, pp. 

 321-330. 



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