142 Scientific Proceedings, Royal Dublin Society. 



is most strikingly illustrated by a coarse leucitic lava exposed near 

 Orchi, on the volcano of Roccamonfina, where leucites, some two 

 or three centimeters in diameter, enclose many large and perfect 

 crystals of sanidine and pyroxene, which, in some cases, are 

 entirely enveloped, or protrude a short distance from their surface. 

 We might, with such a series of contradictions, feel inclined to 

 give up further experiments in the laboratory. Before, however, 

 let us compare what lias been done by the chemist, and see if it is 

 borne out by rocks as presented to our observation by nature. We 

 will commence by recalling the interesting researches of Sir James 

 Hall, 1 who noticed that if such igneous rocks as whinstones and 

 basalts were fused and cooled quickly, a glass resulted ; but by 

 keeping them near fusion-point [recuitol modern French authors), 

 or allowing them to cool slowly, a crystalline structure resulted. 

 These experiments were followed up by Gregory Watt, 2 who went 

 a step farther, and demonstrated that the sp. gr. increased in pro- 

 portion to the prolongation of cooling. 



The absence of microscopical research prevented any important 

 inferences from being drawn from these early experiments, and it 

 was not till the investigations of Daubree, Hautefeuille, Freidel, 

 Sarasin, Fouque, Michel Levy, and others that much advance 

 was made. These authors 3 found that by recuit, more or less 

 prolonged, the following minerals might be obtained from their 

 fused chemical components : — Peridote, pyroxene, nepheline, leu- 

 cite, triclinic felspars, mellilite, gehlenite, and sphene ; whilst 

 from mixtures not corresponding to the mineral obtained the 

 following were prepared : — Tridymite, oxides of iron, and perov- 

 skite. Many of the first group are obtainable from indefinite 

 mixtures. It is this latter point that is undoubtedly the true key 

 to this enigma of the different results in nature, and in the 

 laboratory. 



It will be convenient to take up the principal rock-forming 



1 "Experiments on Whinstones and Lava, 1798 ;" and also Trans. Boy. Soc. Edinb. 

 1805, vol. v., pp. 8 and 56. 



2 " Observations on Basalt and the Transition from the Vitreous to the Strong Tex- 

 ture which occurs in the gradual Refrigeration of the Melted Basalt, with some Geolo- 

 gical Remarks."— Phil. Trans., 1804, p. 279. 



3 Encycl. Chirnique, tome ii., Metalloids. I er Appendice. Reproduction Artificel 

 des mineraux e roches. L. Bourgeois, p. 10. 



