ON THE CRYSTALS IN LAVAS. ] 35 



vapours in the same degree as schoerl ; as their effects upon npon hj vol- 

 it appear to be not inferior to those upon the lava. At least *^^"^*^ f^rues 



^^ _ ^ more easily 



none of the pieces I possess, that have been exposed to than schoexJ 

 their action, afford any leucite in good preservation; though 

 it retains its characteristic form in the midst of red hot 

 lava. When the heat is carried higher, it is capable of 

 softening it, and occasioning it to undergo a sort of calci- 

 nation. It then cracks, and the matter of the lava pene- 

 trates these cracks in the leucite ; whence we perceive within 

 it particles of lava, which are distinguishable by their black 

 or brown colonr, and little blebs: but the form of the leu- 

 cite is preserved, because, as the lava entirel} surrounds it, 

 no part of its surface can separate from it. This is the 

 case with the leucites of the ancient lava of Viterbo ; and 

 on the piece in my possession there are several niches of 

 leucites, with the impression of their facets. The lava and 

 leucites coming together out of the fires of the volcano, as 

 the lava there must he more perfectly fused than when it 

 flows exposed to the open air, and in its subterranean coarse 

 must meet with narrow passages in which it is compressed, 

 its matter must penetrate more easily into the cracks of the 

 leucites. 



It has been said, that no leucites are found in lavas that Not easy to ds- 

 have flowed with rapidity, but that they are confined to ^f"''?*' ""u"' 

 such as have flowed slowly. This is a mere ideal dlstinc- flowed sioiirly. 

 tion : for by what signs can v/e determine, whether a lava 

 have flowed slov/ly or rapidly ? I fancy it would puzzle 

 any man, to determine this with certainty : and besides, 

 what change can the less or greater velocity or slowness of 

 its course occasion in the substance of a lava? 



The following is a very remarkable fact related by Mr. Loose leycit<» 



Dolomieu. ** Loose leucites are so abundant in the vicinity 1 """ "^^^ 



_•' Rome. 



of Rome, that the road from Ronie to Frescati may be said 

 to be covered with them. The rain washes them away, and 

 collects them in vast quantities in the ditches by the road- 

 side." To this fact Mr. Dolomieu saiijoins some conjec- 

 tures respecting the origin and fonuation of the leucites, 

 in which I think he is luistaken, though he is far from sup- 

 posing them to have been formed of the matter of lavas. 



I have not seen this singular place, but I possess a pretty They come 



large 



