402 WEED AND PIRSSON — HIGHWOOD MOUNTAINS OF MONTANA. 



rock whatever is seen. Near the immediate contact of the igneous rock 

 with the sedimentary strata the sandstone beds curve up sharply on all 

 sides. Beyond this, where the trenching by the streams has gone on, the 

 sandstones have been cut into and the intrusive sheets which form a 

 peripheral fringe around the mountain are brought to light. These rela- 

 tions are shown on the map, 

 figure 4, and by the cross- 

 section, figure 6, page 407. 



When we consider the 

 form of the mass presented 

 by Square butte, the ring of 

 upturned sediments around 

 it and a number of other 

 considerations of structure, 

 etcetera, which will be pre- 

 sented later, it is plainly evi- 

 dent that the butte is a great 

 laccolite stripped of its sedi- 

 mentary cover, but not yet 

 sufficiently eroded to lose its 

 general form. This interpre- 

 tation of its origin is also 

 supported by the occurrence 

 to the eastward of Square 

 butte of the laccolite through 

 which the Shonkin Sag gives such a remarkable section, and which has 

 been alluded to on page 399 of part I of this article. 



LOWER ZONE OF DARK HOODOOS. 



Square butte is remarkably impressive, even from a distance. From 

 every point of view it is seen to present, first, a base of dark, somber slopes^ 

 extending nearly half way to the summit, which in turn are capped by 

 light colored ones that over great areas are often a glaring white. Within 

 a few miles of the butte the dark base is seen to be most fantastically 

 eroded into jutting towers and spires of rock, recalling the strange shapes 

 given by weathering to the volcanic breccias and conglomerates of the 

 Absaroka range and other portions of the Rocky Mountains region. 



On approaching nearer the mountain one's attention is still more 

 strongly fixed by these two pecuharities. The eroded base is seen to 

 consist of a vast series of " hoodoos '"^ or forest of strangely shaped mono- 

 liths, which surrounds on all sides the lower slopes of the mountain 

 and which die out at about a given height. 



\SODAUTE SYENITE 

 Figure i.— Geological Map of Square Butte 



CRETACEOUS 



* See Century Dictionary, p. 2877. 



