crystallized Bodies contained in Lava. 300 



pi<lly contain no lencites, and that those which have run 

 out slowly contained them. This is a mere ideal distinc- 

 tion ; because, By what sign can we ascertain that one lava 

 has flowed rapidly and the other slowlv ? We should be very 

 much embarrassed to determine it in a certain manner; and 

 besides. What change can be produced in the substance of a 

 lava bv the more or less rapidity or slowness of its course ? 



The following is a very remarkable fact related by M. Do- 

 lomieu : — '' The isolated leucites are so abundant in the en- 

 virons of Rome, that it may be said tlist the road to Frescali 

 is covered with them ; the rains wash them down and col- 

 lect them in large quantities in the ditches by the side of the 

 road." At the end of this fact M. Dolomieu presents some, 

 conjectures upon the origin and formation of leucites, in 

 which, I think, he is wrong; but he is very far from think- 

 ing that they are formed of the matter of lavas. 



1 have never seen this singular place, but I am in pos- 

 session of a great number of these same leucites, from the 

 sniallest possible size to that of a small cherry ; they seem 

 to come from spongv lavas, not very far distant, which are 

 decomposed. I have seen them of the same nature near 

 Civita-Castellana; all their surface was sprinkled with a 

 multitude of white grains. Unfortunately, and to my great 

 regret, it rained heavily at the moment, so that I could not 

 leave the carriage. How could we conceive that this multi- 

 tude of isolated leucites in the environs of Frescali have been 

 formed from the substance of the lava which contained 

 them? The latter are a little transparent, and of a slight 

 yellow colour. Do we recognise the colour and the sub- 

 stance of the lava in these leucites? In truth, we might 

 maintain, with equal propriety, that the grauates contained 

 in a rock have been formed from the substance of this rock. • 



M. Fleuriau de Bellevue thinks that the crystals throv.'n 

 out in an isolated manner by the crater " aie new product?, 

 which have taken their origin in the cralcr itself on a former 

 cooling." 



There never was formed in the crater, or, to speak more 



precisely, on its interior sides, any thing else than c|C)Slals 



U 4 ' of 



