THE LITERARY 



OF THE LINNiEAN ASSOCIATION OP PENNSYLVANIA COLLEGE. 



Vol. II. JUNE, 1846. No. 8. 



GEOLOGY. NO. IV. 



Igneous action. 



The inquisitive explorer, who every where encounters numerous 

 evidences that changes of the most decided character and on an exten- 

 sive scale have taken place in the structure of the Earth, very naturally 

 enquires what are the probable causes which have been instrumental in 

 their production. His observation and past experience, together with 

 the character of the results under consideration, readily suggest to him 

 that two causes have been principally at work, viz. the agency oi fire 

 and water. The latter degrades hills, mountains, and continents, trans- 

 ports the washed materials to other localities, and thus becomes the o- 

 riginating cause of all stratified rocks. The former acts in a contrary 

 direction; elevating hills, mountains, and continents, and diversify- 

 ing the earth with dry land and ocean, with mountains and plains, and 

 with hills and valleys. As the elevating and disrupting must act ante- 

 rior to the degrading and levelling force, igneous action first claims our 

 attention. 



6. In our inquiries upon this point, our attention is at once arrested 

 by the large number and the extensive distribution of volcanos, which 

 are evidently the channels of communication between large masses of 

 subterranean igneous matter and the upper world. 



Their number. Arago enumerates one hundred and sixty-three active 

 vents as occurring in various parts of the known world. These, regu- 

 larly or at intervals, send forth into the atmosphere smoke, vapor, flame, 

 large stones, sand, scoriae, and a melted mass, or melted rock called lava. 

 Some throw out mud and boiling water. Some are in a constant state 

 of eruption, as Stromboli, Nicaragua, and a few others ; some manifest 

 a state of activity at intervals of a few years, and others only after the 

 lapse of fifty to one hundred and two hundred years, as Etna and Ten- 

 eriffe. Besides these one hundred and sixty-thiee, there are some which 

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