414 REGINALD A. DALY 



It is obvious that the gravity-separation of phenocrysts in lava 

 which has once escaped from its conduit and then cooled rapidly, will 

 not be conspicuous in the average flow. The separation will be most 

 pronounced in the upper levels of an active volcanic neck or lava- 

 filled fissure. 



The rising of phenocrysts in magma more dense than themselves 

 has seldom been actually observed, partly for the reason that few 

 phenocrysts are lighter than their matrix. Mercalli has described a 

 case of the concentration of leucite phenocrysts (sp. gr. at iioo°C. 

 about 2.42) in the surface layer of an 1883 flow of Vesuvian lava 

 (sp. gr. molten at 1100° C. about 2.70).^ An analogous instance is 

 recorded by Morozewicz who found that tridymite (sp. gr. at 1300° C. 

 2.24), derived from converted quartz grains, floated to the top of a 

 crucible in which he had melted two pounds of granite (sp. gr. at 

 1300° C. about 2.40).^ 



3. Andesite actually observed to have been derived from basalt at an 

 active volcano. — But all deductive argument for the hypothesis must, 

 in order to be entirely convincing, be supplemented with the positive 

 facts of the field. The great value of Velain's discovery in the lie 

 de la Reunion (Bourbon) has surely never been adequately recognized 

 in discussions on the origin of magmas.^ A few weeks before his 

 party reached Reunion the 8000-foot volcano at the southeastern end 

 of the island erupted fluid lava at its summit and, simultaneously, 

 other lava issued from a fissure on the flank of the volcano, a fissure 

 which communicated with the main vent. 



The lava issuing at the summit was relatively acid and had a com- 

 position apparently similar to that of the lava collected at the summit 

 in the walls of the crater itself. The latter was analyzed by Velatn, 

 with the following result : 



SiO. 57.49 Na,0 3.64 



AbOj 16.41 K,0 2.92 



FeO 10.65 Loss on ignition . . . 0.13 



MnO 0.06 



MgO 1.23 100.05 



CaO 7.52 Sp.gr 2.79 



1 Atti delta Soc. ital. di scienze nat., Vol. XXVII, 1884. 



2 Tscher. min. u. petrog. Mitt., Vol. XVIII, 1898, p. 332. 



3 C. Velain, Mission de Vile Saint-Paul (Paris, 1888), pp. loi ff. 



