FKAGME^TAL VOLCAKIC ROCKS— 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 sand- 

 stones 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 w^hich 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 unjustifia- 

 ble 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 believ- 

 ing that the bulk of strata presumed to consist of materials erupted in a pul- 

 verulent 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 conclu- 

 sion that the volcanic activity of the High Plateaus was inaugura;ted 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 can- 

 not 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 accu- 

 mulated in local lakelets which may have lingered for a period after the 



