PALISADE BUTTE AND SlIOXKIN SAG. 399 



The rock composing these columns is very dark and heavy and of ba- 

 saltic appearance, consisting largely of augite. It is rather coarsely crys- 

 tallized and is the shonkinite described in the second part of this paper. 



The top of the butte is a wedge-shaped mass of a light colored syenitic 

 rock, the thicker j)ortion turned to the south, where it forms a small 

 clifl'. It is made U{) of rather thin plates. Its exact relation to the dark 

 columnar mass is somewhat uncertain, but as dikes were seen cutting 

 the latter near the light rock it has probably been extruded through it. 

 With respect to the butte as a whole, its relations to the horizontal sedi- 

 mentary rocks through whicli it has been projected upward, together 

 with its own structure and rock character, indicate clearly that it is the 

 denuded remnant of a former volcanic ])lug or core. 



The Shonh'ui Sag Laccolitc. — North of S(piare butte the walls of the 

 Shonkin Sag expose a beautiful section of a laccolite intruded in the 

 sandstones of the Cretaceous series. The laccolitic rock forms massive 

 pillars, whose dark color contrasts strongl}' with the wliite sandstones 

 above and below. The exposed section of the laccolite is a mile and a 

 quarter long and at least 150 feet thick. At each end the laccolite taj^ers 

 in a blunt wedge, the covering of sandstones curving up very al)ruptly. 

 There are several fringing sheets running out from the laccolite into the 

 tilted beds and filling the interspaces formed by the folding. These 

 sheets do not extend far from the main mass. The base of the laccolite 

 is, however, prolonged in a sheet of perhaps 25 feet in thickness, which is 

 exposed as a black band in the light colored cliff extending 5 or 6 miles 

 from the laccolite. 



GEOLOGIC HISTORY. 



The geologic history of the Highwoods may be briefly summarized as 

 follows : 



Volcanic forces breaking througli the horizontal strata of a deeply 

 trenched and partly dissected plateau formed a group of volcanoes 

 whose eruptions, continued at intervals through a considera))le period of 

 time, resulted in the fcjrmation of great numl)ers of dikes and the accumu- 

 lation of fragmental deposits and lava-flows. Subsequent denudation 

 largely removed the materials forming the volcanic cones, exposing the 

 granular rocks filling the old necks, while the remnants of the cones form 

 the greater part of the mountains. To the eastward tlie earlier eruptions 

 formed tlie laccolitic masses of Square l)utte and the Shonkin Sag lac- 

 colite. The group presents certain analogies to tlu; Crazy mountains 

 farther south, but differs from them in having surface flows and breccias. 



With respect to the i)etrographi(; cliaracters of tlie igneous rocks of the 

 Highwood.s, it may be remarked here tliat their study, which was made 

 in the field, together with a preliminary examination made of thin- 



