287 



North of the ravine of Piedras A%ules. I did 

 not see there any pyroxene ; but on the very 

 spot I recognised a number of crystals in the 

 amygdaloid, which appears so strongly linked 

 to the gruenstein, that they alternate several 

 times. 



The geognost may consider his task as ful- 

 filled, when he has traced with accuracy the 

 positions of the divers strata ; and has pointed 

 out the analogies^ which these positions display, 

 with what has been observed in other countries. 

 But how can he avoid being tempted to ascend 

 to the origin of so many different substances, 

 and to inquire how far the dominion of fire has 

 extended in the mountains, that bound the 

 great basin of the steppes? In researches on 

 the position of rocks, we have generally to com- 

 plain of not sufficiently perceiving the connexion 

 between the masses, which we believe to be su- 

 perimposed on one another. Here the difficulty 

 seems to arise from the too intimate and too 

 numerous relations displayed by rocks, that are 

 thought not to belong to the same family. 



The phonolite (or leucostine compacte of Mr. 

 Cordier) is pretty generally regarded by all, 

 who have at once examined burning and extin- 

 guished volcanoes., as a flow of lithoid lava. I 

 found no real basaltes, or dolerites; but the 

 presence of pyroxene in the amygdaloid of Para- 

 para leaves little doubt of the igneous origin of 



