200 Pirsson — Microscopical Characters of Volcanic Tuffs. 



may be found in the same tuff. To determine which origin 

 the crystals of a particular mineral in a tuff may have had, two 

 features should be considered. The first is whether, regarding 

 the general petrologic nature of the tuff, as determined by its 

 association with other igneous rocks in the field, its chemical 

 composition provided an analysis is made, or in lack of it by 

 the character of the general assemblage of crystalline material 

 it contains, such a mineral could be considered one of its 

 normal components. Thus in a tuff which examination had 

 shown to be of trachytic or phonolitic nature, crystals of 

 quartz would have to be regarded as of an origin foreign to 

 the magma and therefore in the second class. The second 

 feature relates to peculiarities which the crystals themselves 

 may show. The ones of the first class, produced in the liquid 

 magma, may be regarded as prematurely born phenocrysts and, 

 like the phenocrysts of the lavas, they may be spongy, filled 

 with inclusions consisting of other minerals, indeterminate 

 microlites or blebs of glass, or contain cavities filled with 

 liquids or gas. They may also be corroded, with deep embay- 

 ments, the latter filled perhaps with glass, as so often seen in 

 quartzes and olivines ; and in addition they may have, especi- 

 ally with feldspars, the clear glassy appearance associated with 

 sanidine, in contrast with the ordinary habit of the feldspars of 

 the granular rocks. On the other hand, fragmental crystal 

 material torn from older, deeper-seated rocks of the basement, 

 or projected by explosion, may be quite lacking in such dis- 

 tinctive properties. The student, however, should guard 

 against thinking that these features can be always used as an 

 invariable receipt for making such a distinction. They may be 

 quite wanting, or the crystals may have been derived from 

 older lavas forming part of the cone, which may have been 

 projected by explosion. But if used with discretion, in com- 

 bination with other circumstances to be described later under 

 lithic tuffs, they may be of great service. In case the crystals 

 show corrosive embayments filled with a crystallized ground- 

 mass, like the quartzes of rhyolites or the olivines of basalts, it 

 may be safely inferred that they have been derived from older 

 lavas. 



Forms of the Crystals. — The forms exhibited by crystals in 

 tuffs may be in places, as is well known, strikingly perfect, 

 furnishing when they are of sufficient size some of the best of 

 cabinet specimens, like the augites from the tuffs near Aussig 

 in Bohemia. With the lens, quite minute ones of perfect 

 development may be observed, and if the tuff is not too in- 

 durated, picked out with a knife or needle point and examined 

 under a low power on the microscope stage. Such crystals, 

 complete in their outward crystal form, must be of the first 



