Pirsson — Microscopical Characters o~f Volcanic Ttiffs. 199 



in those derived from rhyolitic and dacitic magmas ; less 

 frequently from those of trachytic or andesitic composition : 

 and still less so from basaltic ones. This of course depends 

 in great measure on the inversely varying viscosity and crystal- 

 lizability of the different magmas ; the more viscous felsic 

 ones forming glass under conditions where the more fluid 

 mafic ones readily crystallize. The exact magmatic relations 

 of a vitric tuff can only be determined by chemical analysis, 

 but it may be noted that the glass of fresh felsic varieties is 

 usually clear and colorless ; that of basaltic ones colored brown. 

 This applies to them of course only as they appear in thin section 

 by transmitted light and to the glass particles themselves, not 

 to interstitial matter which may be colored, as previously 

 discussed. 



It should be mentioned here that basaltic tuffs, composed of 

 angular or concave edged particles of brown glass, are called 

 joalagonite. They are often vesicular and may enclose micro- 

 lites of plagioclase or crystals of augite or olivine ; often the 

 fragments are outlined by bands of a lighter or different color, 

 which are sometimes isotropic like the glass cores, sometimes 

 weakly birefringent. Such banding indicates a certain degree 

 of alteration. 



Crystal Tuffs. 



While it has happened, and perhaps not infrequently, con- 

 sidering the number of instances which have been observed, 

 that volcanoes have projected material consisting of practically 

 nothing but crystals of a particular mineral, such as augite, 

 olivine, feldspar, leucite, etc.; and even masses of loose 

 titanite crystals are mentioned by Doelter* as occurring on one 

 of the Cape Verde volcanic islands ; tuffs, composed entirely 

 of crystals, must be very rare, and their origin as such might be 

 difficult to determine. On the other hand, crystals of minerals, 

 the kinds depending largely on the nature of the magma, either 

 perfect in form or more or less fragmental, are found in nearly 

 all tuffs ; and when they become a dominating or striking 

 feature of them the rocks may be referred to this division. It 

 may be said, however, at the outset that while crystal tuffs of 

 acidic felsic magmas are common, those of basaltic ones are at 

 least comparatively rare. 



Origin of the Crystals. — -This may be two fold ; they may 

 have originated in the magma itself, which, caught in the act 

 of crystallizing by the explosion, consisted of a mass of crystals 

 mingled with liquid ; or they may have come from disrupted 

 and shattered portions of the rocky walls through which the 

 vent has been drilled. JSTot infrequently a mingling of both 



* Pedogenesis, p. 147, 190G. 



