THE 



JOURNAL OF GEOLOGY 



APRIL-MAY, 1906 



PROWERSOSE (SYENITIC LAMPROPHYRE) FROM TWO 

 BUTTES, COLORADO^ 



WHITMAN CROSS 



In 1896 G. K. Gilbert^ described in this Journal the laccoliths 

 occurring at Two Buttes, in the Arkansas valley, near the eastern 

 border of Colorado and about 150 miles from the mountain front 

 at Canyon City. The igneous rocks of this locality, embracing 

 specimens from two laccoliths and many associated dikes, were 

 submitted to me for determination, but no detailed descriptions 

 have as yet been pubHshed. The interest attaching to the principal 

 rock type and some of its associates made a renewed examination 

 of their field-relationships desirable, and such a study is contem- 

 plated for the field season of the current year. 



The rock of the larger laccoHth at Two Buttes possesses an 

 unusual mineral composition, and it was subjected to chemical 

 analysis by W. F. Hillebrand. The analysis, which was pubhshed, 

 with a brief description by myself, in the well-known compilation 

 of rock analyses made in the U. S. Geological Survey laboratory,^ 

 has led to the classification of the rock by the quantitative system 

 in a subrang without other known representatives until the one 

 described by Mr. Bastin in the accompanying paper was discovered. 

 The rarity of such rocks, and the fact that the one from Colorado 



1 Published with the permission of the Director of the U. S. Geological Survey. 



2 "Laccolites in Southeastern Colorado," Journal of Geology, Vol. IV, pp. 816-25. 



3 "Analyses of Rocks," tabulated by F. W. Clarke, Bulletin No. 228 (1904), 

 U. S. Geological Survey, p. 186. 



Vol. XIV, No. 3 165 



