586 



PORPIIYRIT1C MOUNTAINS. 



granite ; but numerous fissures and indentations are still discover- 

 able. 



Porphyry generally occurs in isolated peaks, with almost vertical 

 flanks ; seldom in continuous chains. Porphyritic mountains, says M. 

 Maury, imprint on the landscape a peculiarly picturesque character. 

 This rock sometimes appears under the form of tall pillars set in close 



THE ORGAN MOUNTAINS OF Rio JANEIRO. 



juxtaposition it is then known as columnar porphyry; and to 

 groups of these columns have been given in some countries the name 

 of Orgues or Organs, on account of their resemblance to the organ 

 pipes which discourse solemn music in our cathedrals. 



Thus : in Mexico two mountains occur distinguished by this ap- 



