332 KEMP AND KXIGHT — LEUCITE HILLS OF WYOMING 



have been emitted in Laramie time, because this would impl}^ their 

 burial and resurrection, an impossibility for surface flows with frag- 

 mental cones. On the contrary, it is reasonable to conclude that they 

 are all post-Bridger. It follows also that not onl}^ are the}" j^^st-Bridger, 

 but they must have been poured out, at least at Zirkel, Emmons, Cross, 

 Osborn, Black Rock, and Orenda mesas, after the Bridger and Green 

 River beds, which once covered these localities, had retreated because 

 of erosion so as to uncover the Laramie where the mesas now appear. 

 It is therefore evident that the}^ are post-Bridger. The question arises 

 as to how much later. 



While it is very difficult to determine the exact age of the eruptions, 

 they are at least late Tertiary. From recent investigations it has been 

 determined that the Oligocene beds that are so prominent along the 

 eastern" base of the Rocky mountains extended westward as far as Green 

 river. Oregon buttes, which are great landmarks south of South pass, 

 are capped with Oligocene, and at an elevation much lower than their 

 crest good remains of a titanotherium have recent!}^ been discovered. 

 This leads us to believe that originally the Oligocene covered a vast area 

 in this region, and that the greater part of these beds had been removed by 

 erosion before the leucitic eruptions, for in no place w^ere these eruptive 

 rocks found higher than the Bridger. It is evident, then, that these 

 eruptions were post-Oligocene, and it must have been well along in the 

 Miocene, if not during the Pliocene. The}^ can not be considered more 

 recent than the Pliocene, on account of the warping of the mesas, which 

 was no doubt done during the great Pleistocene uplift of the Rocky 

 ]\Iountain region. 



One important consideration affecting the time of their intrusion is 

 this : The cones of tuff and breccia which surmount the mesas can not 

 have been very large ones, because, if we continue their present arcs to 

 complete circles, the latter are not great. Yet considerable fractions of 

 the cones still remain. A protracted period of exposure, geologically 

 speaking, would have destroyed these incoherent piles, even in an arid 

 climate. 



Erosion has, however, eaten well back into the sheets, and has greatly 

 dissected them along the northern slopes, and it is evident that very 

 considerable amounts of work have been accomplished since they chilled. 

 The points made above in discussing the Boars tusk also bear on this 

 question. 



FoRMKR Connections of the Mesas 



The question of the former connection one with anotlier of the mesas 

 would naturally arise in the mind of an observer. To some degree they 



