820 G. K. GILBERT 



the Neocene sand brought up igneous rock of the same general 

 type. The majority of the dikes trend approximately at right 

 angles to the strike, so that if produced they would pass near 

 the center of the uplift ; but on the southwestern slope of the 

 dome a small group trend approximately with the strike. 



•In the various characters thus far mentioned the Twin Butte 

 laccolites do not differ from the ordinary type, but petrograph- 

 ically they are rather exceptional, and their peculiarity of rock type 

 is the occasion of geomorphic characters which are equally notable. 



Classifying igneous rocks broadly as basic, intermediate, and 

 acid, most of the laccolitic rocks heretofore described belong to 

 the intermediate group, a smaller number are acid, and basic 

 examples are comparatively unknown. R. C. Hills characterizes 

 as doleritic two small masses observed in Huerfano Park, Col- 

 orado," and Weed and Pirsson have described large laccolitic 

 masses, occurring in the Highwoocl, Little Belt, and Bearpaw 

 mountains of Montana, in which the outer parts are basic and 

 the inner acid."" The Twin Butte rocks also are basic and 

 closely resemble some of the Montana types. As the chief 

 mass is so little dissected that all collections were from the upper 

 part, it is quite possible that the new locality belongs structurally 

 with the type discovered by Weed and Pirsson. 



The specimens collected have been placed in the hands of 

 Mr. Whitman Cross and are to be studied in connection with 

 cognate rocks obtained by myself and others from a great 

 system of dikes occurring in areas to the west and south of the 

 locality under consideration. As the result of a preliminary 

 examination he informs me that they are properl}^ designated 

 syenite-porphyry. The essential constituents are biotite, augite, 

 and alkali feldspars, and as the ferro-magnesian minerals pre- 

 dominate, the rocks are basic. They are porphyritic in struc- 

 ture, and large phenocrysts of biotite are characteristic. 



' Proc. Colorado Scientific Soc, Vol. Ill, Part 2, p. 226, 1889. 



-Bull. Geol. Soc. Am., Vol. VI, pp. 389-422 (1895) ; Am. Jour. Sci., 3d ser., Vol. 

 L, pp. 467-479(1895); 4th ser., Vol. I, pp. 283-301, 467-479 (1896). The authors refer 

 also to descriptions of European localities. 



