IF. Cross — Igneous Bocks in Wyoming. 115 



Art. XYI. — Igneous Rocks of the Leucite Hills and Pilot 

 Butte, Wyoming ; by Whitman Cross. 



[Published by permission of the Director. of the U. S. Geological Survey.] 



Contents. 

 Introduction. 



Rocks of the Leucite Hills. 

 The Rock of Pilot Butte. 



Chemical Composition of the Rocks Described. 

 Classification and Nomenclature. 

 Inclusions in the Leucite Hills Rocks. 



Introduction. 



In 1871, in the course of the geological explorations along 

 the 40th parallel, S. F. Emmons found the first leucite-bearing 

 rock to be discovered on the American continent in a small 

 group of hills in southwestern Wyoming, which received on 

 this account the name of the Leucite Hills. The description 

 of the locality given by Mr. Emmons in the reports of the 

 Fortieth Parallel Survey* is very brief, being based upon 

 reconnaissance observations made before the unusual interest 

 attaching to the rocks of the region had been ascertained. A 

 petrographical description of the leucite rock in question is 

 contained in the report by Prof. F. Zirkelf upon the micro- 

 scopical characters of the rocks of the 40th Parallel collection. 



As far as I am aware no further description of the leucite 

 rocks of Wyoming appeared until J. F. Kemp's communication 

 upon them was presented to the Geological Society of Amer- 

 ica, in December, 18964 The material described by Kemp 

 was much better illustrative of the variation existing in the 

 Leucite Hills lavas than was that examined by Zirkel. He 

 also had specimens of the singular rock from Pilot Butte, a 

 point situated some miles west of the Leucite Hills. Zirkel 

 does not describe the Pilot Butte rock, but a short account of 

 it is given by Emmons.§ The specimens from Pilot Butte in 

 the 40th Parallel collection, preserved in the National Museum, 

 are like the material obtained by Kemp and myself, and Em- 

 mons's megascopical description clearly applies to them ; but 

 his statements of their microscopical constitution indicate that 

 a no longer explicable confusion of thin sections led him to 

 describe the rock of Pilot Butte as a plagioclase-bearing 

 trachyte, whereas it is entirely free from feldspar. 



Major John W. Powell also visited Pilot Butte in the course 



* Vol. ii, Descriptive Geology, 1877, pp. 236-238. 



f Vol. vii, Microscopical Petrography, 1876. pp. 259-261. 



j Published iu Bull. Geol. Soc. Amer., vol. v, 1897, pp. 169-189. 



§ Reports of the Fortieth Parallel Surrey, vol. ii, 1877. 



