Pirsson — Microscopical Characters of Volcanic Tuffs. 195 



excessively minute such vesicles will be broken into smaller 

 fragments by violent collisions in the outrushing cloud of gas 

 and ash, or by weight of superincumbent material after deposi- 

 tion. It is also to be noted that the form of such fragments, 

 with the surface large in proportion to the weight, is an excel- 

 lent one for floating and long waftage by air currents. In 

 thin sections of vitric tuffs, sections of such vesicles, or their 

 fragments, appear mostly as lune- or sickle-shaped pieces of 

 glass, as illustrated in b, fig. 1. It commonly happens also that 

 in the projected particles of magma a number of contiguous or 

 even coalescing bubbles are forming and bursting at the same 

 time. The fragments of the cell walls which are left will 



Fig. 1. 



*% 



€ / 3 h 



Fig. 1. Forms of glass strands and particles in tuff. 



show various cusp-like shapes of the character illustrated in c, 

 fig. 1. A further extension of this leads to cases where, 

 owing to increasing viscosity, or to greater depth from the sur- 

 face in the magma particle, not all of the bubbles are able to 

 burst their cell walls and some remain imprisoned in the glass. 

 The effect is then like d in fig. 1. Some of the spheroidal 

 openings may of course be sections of bubble pits seated on 

 the surface of the particle after a has been blown off. In the 

 rending of the magma it may happen also that particles of it 

 are driven apart while they still have points of attachment; if 

 the magma is not too excessively viscous they will draw out 

 threads of glass after them. Tiny broken pieces of such 

 threads are seen as rod-like bodies, sometimes carrying inclu- 

 sions or drawn-out vesicles ; e, fig. 1. This is the same as the 

 well known Pele's hair. 



Pieces of the character of d, fig. 1 are larger than b, c and e 

 and represent transitions from dust into ash ; as their dimen- 

 sions grow they pass into lapilli. Not infrequently one dis- 



