MUSEUM OF COMPARATIVE ZOOLOGY. 281 



The blackened parts of this brown hornblende are magnetic, and it is 

 probable that much of the magnetite scattered through the groundmass 

 came from the destroyed hornblende. Often these hornblendes, occur- 

 ring in the andesites that have a reddish color, have been torn into 

 ferritic fibres, which have been heaped up' or else scattered throughout 

 the groiuidmass. These changes have apparently been produced by 

 the remclting of pre-existing rocks of the same kind. The augite varies 

 inversely with the hornblende, and in its relations to that mineral leaves 

 no doubt that it comes from a recrystallization of the hornblende mate- 

 rial dissolved in the magma. 



The feldspar and augite are here products of crystallization of the 

 magma. These rocks are subject to alteration, but in a less degree than 

 the basalts, the changes being somewhat similar, and giving rise to 

 several varieties now classed by lithologists as distinct species, some of 

 the most important of which are propylite, porphyrite, hornblende- 

 porphyry, l)lack porphyry, part of the felsite or felsite porphyry, diorite, 

 etc. No distinction can be legitimately drawn between the hornblende 

 and augite andesites, on account of their mutual relations, as given 

 above ; as well might we have olivine and augite basalts. 



We next pass into that most obscurely defined of the volcanic rocks, 

 trachyte, which has been made to include the rhyolites on one side and 

 the andesites on the other. The range of silica here is perhaps from 

 65 to 70 per cent. The base is a light-gray, almost colorless glass, 

 somewhat micro-felsitic ; sanidin begins to predominate, the plagioclaso 

 diminishes, biotite appears, and the hornblende is partly now a min- 

 eral of the first class and partly of the second. Augite rarely occurs, 

 while quartz, which has been occasionally seen as a mineral of the first 

 class in the basalts and andesites, begins to be more abundant. This, 

 like the others, has its old and altered forms. 



Next we pass on to the rhyolites, which contain from about 70 to 

 80 per cent of silica, possessing a clear, glassy base, which is, how- 

 ever, often colored by foreign materials, holding crystals and fragments 

 of quartz belonging to the minerals of the first class, and more abun- 

 dant sanidin, biotite, and hornblende, crystallizing out of the magma. 

 The base is most prominent in the compact rhyolites, and its devitrifi- 

 cation gives rise to the various structures that characterize them. 

 This devitrification gives rise in the older and more altered rhyolites 

 to the feldspar, quartz, and micro-felsitic (so-called) base, that has so 

 puzzled lithologists in the study of the felsites. The rh3'olites of all 

 volcanic rocks pre-eminently show lamination produced by flowing, 



