£4 Troost on the. Pyroxene. 



sublimed or washed away. We may observe in nearly 

 all the craters, and particularly in the solfutara, decom- 

 posed lavas which contain crystals of Pyroxene perfect- 

 ly well formed, also in a state of decomposition. 



The calcinations which the lavas undergo continually 

 around the burning craters, does not act so effectually 

 upon the crystals of Pyroxene. These are often in a 

 perfect and unaltered state at the same time that the la- 

 vas are crumbling to dust. Dolomieu has collected in the 

 crater of Monte Roso, a yellow resinoid Obsidian, con- 

 taining Pyroxene covered with a thin white pellicle ; he 

 collected in the same crater, red scoriae containing the 

 same crystals. This scoriae, which is very brittle, is form- 

 ed by the natural calcinations of the glassy lava; this is 

 easily ascertained by the blow-pipe, which converts this 

 lava into the same scoriae. Dolomieu saw specimens 

 which presented the two varieties. It is besides known 

 that unaltered crystals of Pyroxene occur in vitrified la- 

 va ; it is only by a long continued action of heat that they 

 crack and fall at last to dust. 



Even the other atmospheric agents, only after a vcry 

 long exposure, have a slight action on Pyroxene ; and 

 it is to this circumstance that we ascribe the perfect con- 

 servations of the currents of ancient lava which are yet 

 in existence, and have all the appearance of having been 

 recently formed. The Basalt and other volcanic produc- 

 tions having Pyroxene as a base, are in the same case. 



Pyroxene offers, nevertheless, two different states of 

 decomposition ; in the one, it becomes of a rusty or earthy 

 yellow. It preserves partly its lamillar structure, and is 

 friable. Some naturalists have made of it a species under 

 the name of Limbilite. The lava of Teneriffe, Bourbon, 

 Brisgau, offer instances of this kind ; the decomposed 

 Pyroxene occurs here with perfect unaltered crystals 



