306 KEMP AND KXIGHT LEUCITE HILLS OF WYOMING 



Page 



Former connection of the mesas 332 



The cones 333 



Nature of the eruptive mesas 334 



Secondary minerals 335 



Sand glaciers and dunes 335 



Historical Review 



For a number of reasons special interest attaches to the geological feat- 

 ures of the Leucite hills of Wyoming. The hills present a series of rock- 

 types which, on the petrographical side, are unique and extremely im- 

 portant in the systematic study of the eruptive rocks. The rocks were 

 the first with leucite discovered in America and, except for one occur- 

 rence in the Dutch Indies, the first to be observed outside the continent 

 of Europe. The obvious joj^ of Professer Zirkel in recognizing them, as 

 shown in his classic report on the rocks of the fortieth parallel, still 

 arouses a responsive chord in the minds of the later, and above all the 

 younger, workers who read his pages. They are the onl}^ eruptive rocks 

 known to have original phlogopite, as Doctor Cross has remarked, and 

 a rare hornblende in some of their flows still invites further investiga- 

 tion. 



The petrographical characters, however, are not their sole claims to 

 interest. They constitute a little group of volcanoes far removed from 

 any neighbors of like kind and rise like landmarks amid the arid plateau 

 of the Red desert of W3'oming. Their sheets, cones, necks, and dikes 

 afford singularly interesting phenomena, to which no writer has thus 

 far seriously addressed himself. Even the period of their outbreak has 

 not as yet been demonstrated in print. No complete record of the num- 

 ber of exposures and no description and discussion of the individuals 

 have hitherto appeared. It is to these structural and stratigraphic fea- 

 tures that the present writers purpose to specially address themselves, 

 but in the detailed study new exposures have been found involving 

 some undoubted dikes, and some new petrographical notes have resulted- 

 We also desire to make available a fairh^ detailed geological and petro- 

 graphical guide, so that our successors ma}^ intelligentl}' visit the hills 

 and collect among them. 



Previous Papers 



The Leucite hills have been already mentioned five times in print. 

 S. F. Emmons* rode up to the southern mesas and gathered the speci- 



*S. F. Emmons : Geol. Survey of the Fortieth Parallel, vol. ii, p. 230. 



