PRESIDENTS ADDRESS. NVII. 



axigite is sometimes contained enclosed in the lencitc is a 

 difficulty, the explanation of which one experiment showed to be 

 that tlie inclusions were shut in as liquid portions wliich, during 

 tlic later cooling, had time to ciystallize. In fact, the order of 

 scpjration out of a mass in purely igneous fusion appears to be 

 the natural one, viz., the inverse of the order of fusibility. 

 Hence, when two felspars occur in a rock, the larger crystals 

 are frequently of a different species from the crystals of the 

 ground mass. If the former are auorthite, the latter are frequently 

 labradorite or oligoclase, while the reverse order seems not 

 to have been observed. 



Scarcely less interesting arc the failures. Neither quartz, 

 mica, orthoclase, nor hornblende could be reproduced, and it 

 would seem, therefore, that rocks containing these minerals were 

 formed under other conditions than that of simple igneous fusion. 

 A. crystalline form of silica was indeed obtained, but it was not 

 quartz, and had separated out from the mass before any other 

 constituent, and at a temperature near that of melting platinum. 

 Hornblende and oligoclase in about the proportions occurring 

 in hornblende andesites formed an augite andesite. 



Very important, too, are the results obtained in attempts 

 to produce the various members of the felspar group inter- 

 mediate between albite and anorthite. Their value lies in the 

 fact that the true nature of these intermediate members is 

 disputed. On the one side Tschermak considers that albite the 

 pure soda felspar, and auorthite the pure lime felspar, are 

 isomorphous, and that the other members are mixtures of these 

 in varying proportions. The optical phenomena of a great 

 number of specimens have been observed, and the composition 

 determined on this supposition by Max Schuster. The other 

 view is that oligoclase and labradorite are independent mineral 

 species in exactly the same degree as the others, and this is the 

 conclusion to which the experiments referred to would seem to 

 point. Mixtures of albite and anorthite were taken in varying 

 proportions, but the only resulting felspars were oligoclase and 

 labradorite, with sharp distinctions in their optical characters. 



