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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 the Qquichua language, called com- 

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

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

 serves as a iamho, or caravansary. 



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