Pirsson — Microscopical Characters of Volcanic Tuffs. 205 



thing their loose porous texture permits the ready jpassage 

 through them of meteoric waters carrying substances in 

 solution, which may attack them and at the same time by 

 deposition introduce foreign substances into them, which in 

 many cases serves to cement them into more or less firm rocks. 

 For another thing, the glass, which may be a prominent or 

 even dominating ingredient, is a substance chemically in 

 unstable equilibrium and therefore ready on the slightest 

 pretext to alter into other and more stable forms of matter. 

 The same is true, only in lesser degree, of many of the crystal- 

 line minerals they contain, in that these compounds have 

 formed originally, and are in equilibria, under a very different 

 set of physical conditions from those in which they are sub- 

 sequently placed. In this latter respect tuffs, however, are not 

 of course different from any of the crystalline igneous rocks. 

 In addition to these causes of alteration tuffs, like all other 

 rocks of whatever origin, may be subjected to metamorphism, 

 both contact and general, and may be more or less modified 

 and even so profoundly that no trace of their original charac- 

 ter may remain. To undertake to discuss fully all such possi- 

 bilities would be almost equivalent to a treatise on the general 

 subject of metamorphism and far beyond the limits of this 

 article. It is proposed here, therefore, only to sketch some of 

 the more important features of these changes and the results 

 that follow from them, with special reference to the tuff nature 

 of the material operated upon. These broad outlines must be 

 regarded by the student as indications of the paths to be fol- 

 lowed in the investigation of particular cases. They may be 

 discussed under the following general headings : Weathering 

 and Consolidation / Devitrification / Contact Metamorjihism ; 

 and General Metamorphism. 



Weathering and Consolidation. 



Weathering and consolidation are not of course one and the 

 same process and yet where the first is taking place the second 

 in a measure, in depths below, must be also occurring. The 

 weathering of tuffs is comparatively easy where they are 

 loose and uncompacted, owing to the ready access of air and 

 moisture and also to the relatively large surface areas exposed 

 by such fine grains. Hence the felclspathic tuffs are readily 

 kaolinized and converted into soft earthy masses. One of the 

 earliest indications of a change in tuffs, when examined in sec- 

 tion, is the deposition in them of some form of hydrated silica, 

 opal, chalcedony, etc. This has been already mentioned in the 

 description of the vitroclastic tuff from Castle Mountain. 

 Every stage of this saturation of the felsic tuffs by hydrated 



