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Show FEAGMENTAL VOLCAJSTO EOCKSâ€"TUFAS. 73 in reality hold among their ingredients a notable percentage of intermingled grains and silt derived from the denudation of sandstones or other quartzif-erous rocks. Thus, these tufas would seem to be nothing more than sandstones and shales of the ordinary kind, so far as their mechanical characters are concerned, and having the same genesis as any clastic strata, but the materials of which they are composed being derived from volcanic instead of from foliated common rocks. On this view of the case there is no apparent reason why they should be sharply distinguished from other strata. It would, indeed, be unjustifiable to proceed to the conclusion that in other parts of the world the so-called tufas have all had a similar origin, for there is abundant reason for the belief that considerable deposits of real "volcanic ashes" exist elsewhere But if the tufas of the High Plateaus are similar to those which in other regions are supposed to be accumulations of ashes, there is reason for believing that the bulk of strata presumed to consist of materials erupted in a pulverulent form has been greatly overestimated, and that such strata, instead of being common, are on the whole rare and of insignificant magnitude. Especially I am confident that these beds do not lead at all to the conclusion that the volcanic activity of the High Plateaus was inaugurated by the ejection of vast bodies of ashes. They seem to point much more logically to the conclusion that eruptions of lavas not now discernible or identifiable took place before they were laid down, and were broken up and wholly or partially dissipated to furnish their materials. These finer deposits rest upon the Eocene beds, which in the southern part of the district I have inferred to be of the age of the Bitter Creek beds of Powell. Whether they are conformable or not is a question I cannot answer. No unconformity has been discovered, both series being very nearly horizontal wherever they are seen in contact It is not certain that the tufas are immediately consecutive in age to the Bitter Creek beds, but at all events I incline to the opinion that no great interval of time separates them. It is an interesting point whether these tufas were deposited before the final recession northward of the great Eocene lake, thus representing the last strata deposited upon this part of its ancient basin, or were accumulated in local lakelets which may have lingered for a period after the |