EPOCHS OF EEUPTION— PROPYLITES. 57 



deposits still remain buried beneath the imn«ense 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 age — are, in fact, the latest strati- 

 fied masses of the region. On the other hand, they must have been depos- 

 ited 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 sev- 

 eral 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 con- 

 tact 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 excava- 

 tions. 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 erup- 

 tive centers are situated in the grand gorge of the Fish Lake Plateau, and 



