308 KEMP AXD KXIGHT LEUCITE HILLS OF WYOMING 



tion and incidentally concernino- the general classification of all the 

 leucite rocks. Doctor Cross gives but a brief outline of the structure and 

 field occurrence. He appears to have regarded the separate mesas as 

 having been once continuous and having been separated by erosion. 

 Doctor Cross only visited the exposures, which we call Zirkel mesa, 

 Orenda mesa, the Boars tusk, Pilot butte, and one that he mentions as 

 North Table butte. This may be the one called by Emmons and by us 

 North Pilot mesa. It is onl}^ fair to state further that his field and labo- 

 ratory observations were made some years before those of J.«F. Kemp, 

 although unknown at the time to the latter, and delayed in publication 

 because of other work. 



Finally the present writers both felt the interest and desirabilit}^ of 

 more detailed study, description, and illustration. One of us (W. C- 

 Knight) has made several trips alone, and both of us together spent 

 some days camping among the mesas and buttes the past summer, 

 studying, photographing, and collecting. We have discovered several 

 exposures not previousl3^ observed and have endeavored not to overlook 

 any important points connected with their structural relations. 



Varieties of Rock 



Professor Zirkel received specimens collected by Mr Emmons from the 

 extreme eastern or southeastern end of the exposure which we call 

 Zirkel mesa. In them he discovered onl}^ leucite, diopside, light brown 

 biotite, and apatite. When, however, one of us (J. F. Kemp) came to 

 examine a suite of specimens from widely separated points, varieties 

 with abundant sanidine were observed and some that departed in the 

 small amount of leucite from the type described b}^ Professor Zirkel. 

 Mr Emmons had also recorded trachyte from North Pilot butte, with 

 no leucite. Doctor Cross named the original variet}^ with abundant 

 leucite " wyomingite ; " the variety with little leucite, but with much 

 sanidine and a rare hornblende, orendite; and the variety which con- 

 sists of diopside, phlogopite, and apatite in an isotropic base, madupite. 

 He explained the determination of trachyte by Mr Emmons as due to a 

 now inexplicable confusion of slides. 



Our observations show, however, that there are several difi*erent flows 

 present in nearh^ all the mesas, and that the rock varies much in the 

 same flow. From the same mesa and from the same flow two of the 

 types, wyomingite and orendite, ma}^ be obtained, and as they can only 

 be discriminated under the microscope they cannot be accuratel}^ col- 

 lected in the field except from established ledges or flows. We have 

 only found madu])ite at Pilot butte. Under the microscope it is obvioush' 



