. Bulletin of the Geological Society of America. Geology. 402 WEED AND PIRSSOX—HIGH WOOD 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
. Bulletin of the Geological Society of America. Geology. 402 WEED AND PIRSSOX—HIGH WOOD 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 peculiarities. The eroded base is seen to consist of a vast series of " hoodoos "* or forest of strangely shaped mono- liths, which surr
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