184 ON THE CRYSTALS IN LAVAS. 



schoerls and chrysolites, since these three crystals are found 

 together in the same lava, separated from each other, and 

 from the matter of the lava, by a line as clear and distinct 

 as separates the small pebbles, that compose a pudding- 

 stone, from the cement that envelopes them ? If one of 

 these crystals be foreign to the lava, so are the other two: 

 this is a natural consequence. The fact assuredly is, that 

 they are all three foreign to it. 

 Argument The two instances I have mentioned of isolated leu- 



fgneous forma- ^^^^^ enveloping pyroxene schoerls are inexplicable facts, 

 tioa. on the hypothesis of these crystals having been formed in 



the igneous way : while nothing is more common, or morie 

 easy to conceive, than such combinations between crystals 

 of different kinds formed in the humid way. 

 Patrin's new « J should never have done,'- says Mr. Fl. de Belle\'ue, 

 ral History. " if I ^'^'^re to bring forward all the objections, that offer 

 themselves to the system of the preexistence of crystals in 

 lava. Several will be found at the articles Lavas and Leu- 

 cites, in the new Dictionary of Natural History, in which 

 Mr. Patrin has strongly combated these suppositions." 



We should re- | ^m sorry to learn this, since the readers of that Dic- 

 gavd facts, nqt . "^ . ^i- ,, ii 



suppositions, tionary, who are aesu'ous or knowmg what lavas and leu- 

 cites are, will be led into errour. T have exhibited facts, 

 and not suppositimis. In the physical phenomena of our 

 globe, the accurate knowledge of which depends always 

 on matters of fact, I never was fond of suppositions, which 

 seldom fail to lead us into some mistake, 

 la the lava of L^t ^^e remind Mr. Fleuriau de Bellevue of a very r«-. 

 cites calcined, markable lava of the ancient volcanic mountain of Viterbo. 

 This lava contains a multitude of leucites from the size of 

 a large pea to that of a rapeseed. These leucites have 

 undergone a kind of calcination, which renders them very 

 white, and the lava that includes thena is black, which oc- 

 casions a striking contrast between these substances. No- 

 thing more strongly marked can be conceived. Now is it 

 cot evident, that all these leucites existed before the lava ? 

 If we dispute this conclusion, we may as well deny, that 

 any foreign substance whatever, included in a rock, has 

 existed before the rock. 

 jUncites 3ctpd Leucite does not resist the action of volcanic fires and 



vapours 



