194 Pirsson — Microscojyical Characters of Volcanic Tuffs. 



demands consideration. They may differ also in their petro- 

 genic nature, the chemico-mineralogical differences depending 

 on the kind of magma; whether rhyolitic, trachytic, andesitic, 

 or basaltic, etc., and these also require attention. We will 

 commence with the simplest type, the fresh vitric tuffs. 



Vitric Tuffs. 



Formation of Tuffs. — Since tuffs are produced by the sud- 

 den violent expansion or explosion of gases in a more or less 

 viscous magma it may be well to consider this phenomenon for 

 a moment. It is obvious, in view of the quantities of ash and 

 dust that are almost instantaneously produced, sometimes 

 reaching vast proportions, that it cannot be the result of bub- 

 bles merely passing out from, or forming at, the surface of the 

 liquid melt. They must indeed be produced simultaneously 

 through a body of the magma which is in the conduit. There 

 is some analogy here to the sudden liberation of steam, on the 

 relief of pressure by overflowage from the pipe of a geyser, 

 occasioning its discharge. But it differs from that of a gey- 

 ser in that in the latter there is only one medium, the water; 

 while in the magma there are the volatile gases and the non- 

 volatile more or less viscous silicate melt. If the sudden pro- 

 duction of the gas bubbles does not take place with expansive 

 force sufficient to rupture the magma, the latter would expand, 

 sometimes enormously, in volume, and a pumice or rock froth 

 would result. Where ash and dust are formed the rending of 

 the magma must occur, and the more complete this is the finer 

 the resultant product will be. The expansion of gas and rup- 

 turing begin in a liquid medium ; the resulting product falls as 

 a rigid glass. We do not know of course the exact march of 

 events between these points; but it is clear that the magma 

 ruptures into separated masses of different sizes, as the volume 

 of mingled gases and molten glass rush out of the conduit. 

 The separate masses are themselves swelling and flying apart 

 into smaller ones as they ascend, and this continues until the 

 stiffening of the glass and the lessening of the expansion of 

 the gas through cooling ■ bring the process to an end. This 

 carries us to the point where we may consider, the forms of 

 the resulting particles. 



Forms of Particles. — A bubble of gas expanding on the 

 surface, either of the original magma or of one of the projected 

 masses, forms a vesicle or bulb, whose wall is thickest at 

 the top and whose sides become thinner and thinner as it 

 expands away from the surface, until they are finally ruptured 

 and the bowl-shaped body is driven from the parent mass. 

 Such a form is shown in a of fig. 1. It is evident that unless 



