30S Observations 'upon the 



tural history, the exact knowledge of which alwavs depends 

 upon truths in point oi fact, I was never fond of supposi- 

 tions, which nirelv lead to any thing; else than error. 



I shall call to M. Flevuiaii de Bellevue's recollection a 

 very rcmaikahle lava of the antient volcanic mnnntain of 

 Vituba. This lava contains a multitude of leucites, from 

 the size of a large pea to that of a grain of rapeseed. These 

 Jeucites have undergone a sort of calcination, which renders 

 them very white; and the lava which contains them is hlack ; 

 which makes a very singular contrast between the two sub- 

 stances. Is it not evident that all these leucites existed pre- 

 viously to the lava ? If we rcicct this conclusion, we might, 

 with equal propriety, contend that every foreign body what- 

 ever contained in a rock has not existed previous to that 

 rock. 



The leucite does not resist the action of fire and volcanic 

 vapours in the same degree as the schorl does : the volcanic 

 vapours seem to have almost as much hold upon it as upon 

 the lava ; at least, none of the specimens in my possession, 

 upon being exposed to their action, showed any leucite well 

 preserved. But it retains its characteristic form in the midst 

 of burning; lava. When the heat is carried to a higher de- 

 gree, it may soften it, and cause it to undergo a sort of cal- 

 cination ; it then falls into shivers, and the substance of the 

 lava penetrates the leucite by the fissures; whence it happens 

 that we see in its interior small parcels of lava, which are 

 distinguished by their black or brown colour and small bub- 

 bles : but the form of the leucite is preserved, and is not at 

 all altered, because, the lava enveloping it intimately, no 

 part of its surface can be separated from it. This is the case 

 of the leucites of the cUl lava of Vltcrba; and we see upon 

 the piece I have," several indentations of leucites with the 

 impression of their faces. The lava and the leucites coming 

 trioether from the lires of the volcano, the lava o^ight to be 

 there in a degree of fusion greater tlian w'l-icn it flows out ; 

 and in its subterraneous route moving througl) narrow pas- 

 sages which compress it, its substance ought to penetrate 

 more easily into the crevices of the leucites. 



l\ has been said that the lavas which have flowed out ra- 

 pidly 



