448 J. E. WOLFF; — GEOLOGY OF THE CRAZY MOUNTAINS. 



Paleozoic ; but neither in the field nor in the laboratory was any structure 

 detected different from that of the rock under normal conditions. 



The monoclinal buttes developed along either side of the range are very 

 striking, especially on the western side. They owe their present elevation 

 to the sheets of intrusive rock, which dip inward with the strata toward 

 the range at varying dip angles in the different localities. The most im- 

 portant of these is the group of three high outlying buttes north of the 

 head of Shields river (''Three Peaks" on the map), which are arranged 

 i n echelon on a mirth-south line, the crest lines of the two northerly ones 

 lying progressively east of the third or southerly one. The strata dip 

 about 30° eastward, and the three buttes have high cliffs facing westward 

 and gentle dip slopes eastward. The crests are formed by heavy sheets 

 or laccolites of intrusive rock from 250 to 100 feet thick, with minor 

 sheets at intervals below, interstratified with the shales and sandstones. 

 These master sheets bulge in the crest of the ridge, maintaining their 

 thickness for about a mile in the case of the northern and southern buttes, 

 and then rapidly thin out to a comparatively narrow bedded dike. Ac- 

 companying this diminution in size the crest of the ridge drops, and the 

 next ridge, formed by another bulging sheet at a different horizon, begins, 

 culminates, and dies out in the same way. This peculiar topography 

 seems therefore due to the intrusion of bulging sheets (laccolites) at 

 different levels in the horizontal strata, the major sheets not having 

 their centers in the same vertical line; the whole complex was then 

 tilted and eroded, the soft shales and thinner sheets being quickly taken 

 off, leaving the master sheets standing. In the Henry mountains Mr. 

 Gilbert has described the first conditions of intrusion without subse- 

 quent tilting. 



Another peculiar type is found in the outlying butte near Martinsdale, 

 on the northwestern edge of the range. This has an oval form, is about 

 two miles long, and has an elevation of 600 or TOD feet above the plain 

 at its base. It is fringed by a line of high cliffs facing outward, through 

 which the interior drainage has cut an outlet. The top forms a basin 

 with gently sloping sides. The Cretaceous strata are found around the 

 base dipping gently inward, while the slopes and crest are formed by a 

 great capping eruptive sheet and at least one lower sheet, with thin inter- 

 vening beds of shale. The thickness of the great sheet was estimated at 

 365 feet and of the lower 150. In the center of the basin, on the summit, 

 erosion has cut nearly through the main sheet, leaving "tall pinnacles of 

 the eruptive rock standing in groups (sometimes 50 feet high), which 

 are due to the perfect vertical prismatic structure of the sheet. Loose 

 pieces of metamorphosed shale found on the surface at the highest point 

 seem to lie remnants of the original covering of the laccolite. The erup- 



