| OCR Text |
Show EPOCHS OF EBUPTIONâ€"PROPYLITES. 57 deposits still remain buried beneath the immense outpourings of middle and later epochs, or whether they have been wholly dissipated, it is impossible to affirm. The period during which these tufas were stratified must be referred to the latter part of the Eocene. They rest everywhere upon beds, which are either of Bitter Creek or Green River ageâ€"are, in fact, the latest stratified masses of the region. On the other hand, they must have been deposited before the final desiccation of the great Eocene lake, which appears to have taken place throughout that part of its expanse now covered by the High Plateaus after the middle and before the close of the local Eocene. They are widely distributed, and could not very probably be supposed to have accumulated in local temporary lakelets. Thus, then, the opening of the eruptive activity goes back into Eocene time. The oldest massive rocks of volcanic origin are found in but few places. The tabular masses which now front the long valleys with escarpments several thousands of feet in height have been scored by ravines, which cut into their innermost recesses. Here, with thousands of feet of more recent lavas and conglomerates above them, are found large bodies of propylite and hornblendic andesite, the former clearly the more ancient of the two. The propylitic masses appear to have been much degraded by erosion before the eruption of the andesites, for patches of conglomerate with water-worn propylitic fragments are overlaid by masses of andesite, and the contact of the two is often of such a nature that there can be no doubt that the massive propylites were water-carved before the andesites were erupted. It is impossible to say anything concerning the extent of these most ancient emanations, for the later rocks have completely buried them, and all that can be seen are the few exposures laid bare by recent faults and excavations. Two centers from which these rocks came have been determined, and they are also found in two other localities, but under circumstances which render it quite possible, and perhaps probable, that the two latter are connected with the two former, the continuity being lost beneath later accumulations. The two eruptive centers are located, respectively, in the northern and southern portions of the Sevier Plateau. The two exposures exhibiting propylitic rocks, which may have been derived from these eruptive centers are situated in the grand gorge of the Fish Lake Plateau, and |
