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which sometimes resemble a spider's web*. 

 We may admit, that these differences consist 

 principally in the degree of heat of the volcanic 

 fire, in the pressure under which this fire acts, 

 and in the nature of the rocks altered by it. 

 Above all, the pressure, which the obsidians un- 

 dergo in their fusion, explains why these sub- 

 stances, except some varieties which I collected 

 near Popayan, are never found whitened. Those 

 of the pumice stones that have the appearance 

 of being formed at great depths, are fibrous, of 

 silky lustre, which abound more in mica than in 

 feldspar, and in which, on the Andes, blocks of 

 eight or ten toises in length have the fibres ex- 

 actly parallel with each other, and perpendicular 

 to the direction of the strata. Several volcanoes 

 too do not throw out any pumice stone ; and 

 those that do, eject them only by their crater, 

 after the flowing of the lavas. Several mineral 

 ogists think, that primitive granular rocks may 

 be changed progressively, and ill their place, 

 either by the fire, or by a penetration of hot and 

 acid vapors, into porphyroidal masses, of a foli- 

 ate or fibrous texture, This opinion seems 

 supported by the existence of the fissured and 

 fibrous feldspars, which we found in the trap- 

 pean porphyries of Quito. These crystals re- 



* Bory d« St. Vineeal, Voy. aux lies d' Afriqiae, t. iii, p. 

 60. 



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