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What in the shelly or Neptunean rocks is 

 owing* to the action of the waters, appears to be 

 sometimes in the volcanic rocks the effect of 

 gaseous emanations* which act in the direc- 

 tion where they find the least resistance. When 

 melted matter moves on a very gentle slope, the 

 great axis of the cavity formed by the disen- 

 gagement of elastic fluids is nearly horizontal, 

 or parallel to the plane on which the movement 

 of transition takes place. A similar disengage- 

 ment of vapours, joined to the elastic force of 

 the gasses, which penetrate strata softened and 

 raised up, appears to give sometimes a great ex- 

 tent to the caverns, which are joined in the tra- 

 chytes or trappean porphyries. These porphy- 

 ritic caverns, in the Cordilleras of Quito and 

 Peru, bear the Indian name of Machays ^. 

 They are in general of little depth, lined with 

 sulphur, and differ by the enormous size of 

 their openings from those which the volcanic 



* At Mount Vesuvius the Duke of Torre showed me, in 

 1805, in currents of recent lava, cavities extended in the direc- 

 tion of the current, six or seven feet long and three feet high. 

 These little volcanic caverns were lined with ironglance, 

 which cannot be called oligiste iron, since Mr. Gay-Lussac's 

 last experiments on the oxides of iron. 



+ Machay is a word of theQquichua language, called com- 

 monly by the Spaniards the Inca's language. Thus Callanca- 

 machay means " a cavern as large as a house," a cavern that 

 serves as a iambo, or caravansary. 



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