198 Pirsson — Microscopical Characters of Volcanic Tuffs, 



effect. In other cases the films are ribbed, as illustrated in o, 

 fig. 3, indicating the cell walls of other bubbles. Sections 

 through such fragments shown in fig. 3 would obviously yield 

 shapes like some of those seen in the tuff, fig. 2. 



In some of these dusts there are great numbers of glass 

 fragments of entirely irregular, haphazard forms and there may 

 be some question regarding the influence of the viscosity of 

 the magma, at the moment of explosion, on the shapes of the 

 particles produced. For with increasing stiffness the less 

 opportunity there would be for normal bubble expansion and 

 the formation of vesicular structures, and the more sudden 

 and complete would be rending and shattering of the glass} 7 

 melt, with the production of irregular fragments. This would 

 also be more likely to happen in highly siliceous magmas, 

 since they are the more viscous ones. In addition it may -be 

 noted that the greater the distance a volcanic dust is carried 

 by air currents from its source, the more purely glassy it will 

 be, since the higher specific gravity of any crystalline material 

 will tend more and more to sift the latter out. 



Vitroclastic Structure. — The particular type of microstrnc- 

 ture which distinguishes vitric tuffs and is illustrated in fig. 2 

 deserves a special name. It is as characteristic for these 

 rocks, both when fresh and when altered, as certain textures are 

 for some igneous ones, or particular structures are for certain 

 organisms occurring as fossils in the sedimentary rocks. It 

 has been designated " ash structure " by Miigge,* a term also 

 used by Rosenbusch : but there are objections to this name in 

 that volcanic " ash," as the term is commonly used, indicates 

 coarser material than that which composes the dust that 

 characteristically exhibits this structure. Most " ash " is 

 really composed of fine lapilli, is largely crystalline, and under 

 the microscope is a very fine breccia. If the structure is seen 

 in rocks composed of ash it is in the finer portion composed of 

 dust that fills in between these micro-lapilli. Hence it is 

 proposed to name it the vitroclastic structure, a term whose 

 meaning is sufficiently obvious and whose faulty construction, 

 like that of vitrophyre, from the standpoint of etymology, in 

 that it is composed of roots from two languages, it is hoped 

 may be forgiven since both components are so well known to 

 all petrographers. There is a suggestion of the sedimentary 

 rocks also in the term, which is not bad, since the tuffs often 

 form gradations between that great group and the igneous 

 ones, as previously mentioned. 



Magmatic Relations of Vitric Tuffs. — While many excep- 

 tions to this general rule may be found, it is true that the most 

 frequent examples and largest masses of vitric tuffs are found 



*Op. cit. 



