808 Olseriations upon the 



itself has no relation to^them, either in its nature, colmir, 

 nor chemical properties, as we shall soon see. 



The form of the leucites and pyroxene schorls is perfectly 

 determined ; nothing is confused, all is precise and well cha- 

 racterized. The leucite is constantly of a round form, shaped 

 into 24 trapezoidal faces, and of a whitish gray colour ; the 

 pyroxene schorl is an octacdral prism with two biedral* py- 

 ramids of a deep olive colour, and sometimes black ; the cry- 

 solite is of the colour of a peridot. These three crystals are 

 found in the spongy and cellular lava, as well as in the com- 

 pact lavas. 



The schorl is so strongly united to the lava that it cannot 

 be detached from it, nor appear with the f>olish of its faces 

 and the distinctness of its angles, unless by a chemical opera- 

 tion produced by the sulphurous acid fumes of the volcano. 

 The leucite is more easily separated, leaving upon the lava 

 a distinct impression of its round shape. Its impressions in 

 the lava mav be compared to those left by granates, cubical 

 martial pvrites, and several other crystallized substances upon 

 the rocks which contain them ; with this difference, that the 

 impression of the leucites is made in a manner in fusion, 

 and that of the granates and pyrites in a rock which had been 

 in the state of softness bv the humid way. 



From this the following conclusion results : That the leu- 

 cites are no more a formation produced in the lava at the 

 moment of its cooling, than granates and pyrites are a forma- 

 tion extracted from the substance of the rock which contains 

 them when it is dried and hardened. Both equally are fo- 

 reign to the substance which contains them, and have existed 

 before it ; the leucites before the lava, and the granates and 

 pyrites before the rock which envelops them. We ofteji 

 found also the leucites isolated, and in a great number, 

 among the volcanic ashes. 



Do wc see in this d.xact description of facts any resem- 

 blance, any analogy between bodie/ crystallized in lavas and 

 the confused heaps of vitreous crystallites formed of the eub- 

 stance of the glass in the crucibles of glass-works ? Neither 

 ^ is there any resemblance between these curious crystalliza- 



* T^i.> is 'Jhe aurhoi'i own term.— A. T. 

 4 tion«; 



