METAMOEPniSM OF TUFACEOUS DEPOSITS. 



247 



proximity of heated magmas or the prevalence of high temperature within 

 a mass of strata are not the only conditions requisite for the activity of that 

 process. Strata traversed by eruptive dikes are sometimes altered for 

 many hundred feet from the contact and sometimes are wholly unaffected. 

 This fact alone indicates that something besides high temperature is re- 

 quired to produce such an alteration. Nor do all the conditions appear to 

 be fulfilled when strata containing suitable constituents are subjected to a 

 high temperature, for cases are common where rocks so constituted and con- 

 ditioned are not altered. Although we do not know all the requirements of 

 metamorphic action, we may feel confident that they are somewhat complex 

 and numerous. One inferential condition is that of a high degree of molecu- 

 lar mobility in the constituents, whereby a free interchange of molecules 

 among the clastic particles or fragments is made possible But precisely 

 how this is effected is a matter of conjecture. It may be by the permea- 

 tion of heated waters or other liquid or vaporous solvents which may not 

 require a very high temperature, and which may even be effectual at quite 

 moderate temperatures. How far Ave are required to postulate the absorp- 

 tion of foreign constituents (alkalis and earths) by the entire metamor- 

 phosed masses or the elimination^' constituents which the masses origi- 

 nally contained are problems too conjectural in their nature for present 

 discussion. That the tufas of East Fork Gallon should have been meta- 

 morphosed while the Tertiary (?) strata vipon which they rest are wholly 

 unchanged is not a matter so wholly surprising. In the former beds all the 

 conditions precedent have been satisfied, in the latter they have not. 



An examination of the heliotypes (V and VI) will show one member 

 more massive than the others which is about 120 feet in thickness. Under 

 ordinary circumstances this would have been pronounced an eruptive sheet 

 without much hesitation. But such a decision would raise some difficult 

 questions. Other layers much thinner, and in some cases not exceeding one 

 or two feet in thickness, are composed of rock very similar to it. Others 

 show a transition from material apparently identical into unaltered or very 

 little altered tufa. In most of the beds rolled pebbles are found, and as the 

 varieties become more and more metamorphosed these pebbles become less 

 and less distinct: and in the massive sheet itself some of these pebbles may 



