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Show METAMORPHISM 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 required 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 conditioned 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 molecular 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 permeation 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 we are required to postulate the absorption of foreign constituents (alkalis and earths) by the entire metamorphosed masses or the elimination of constituents which the masses originally contained are problems too conjectural in their nature for present discussion. That the tufas of East Fork Canon should have been metamorphosed while the Tertiary (?) strata upon 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 |