THE CEAGS. 5 



in the soft rock. Often the boundary between gneiss and rhyolite is 

 defined by small drainage ways worn along the contact. 



Along the western side of the gneissic area the rhyolitic lava rises to 

 altitudes of 8,000 to 8,200 feet, while at the head of Maple Creek it rises 

 to 8,700 feet, the level of the divide between this creek and Grayling 

 Creek. From this it seems highly probable that the lava flooded the valley 

 of Grayling Creek at the time of its eruption, and has since been removed 

 by erosion. The absence of any remnant of rhyolite within this valley, 

 as the map represents, is not based on a careful examination of the valley, 

 but expresses our ignorance in respect to its occurrence there. 



The marked contrast between the topographic character of the 

 southern side of Crowfoot Ridge and that of the northern side is note- 

 worthy. The southern slopes are almost free from lateral spurs of any size 

 and the ridge is approximately straight. On the north, spurs branch off at 

 short intervals, increasing in size toward the west until they attain the 

 proportions of mountain ridges. The three most prominent of these spurs 

 trend north. 



This contrast in topographic configuration is to be explained by the 

 position of the former covering of sedimentary rocks, which were removed 

 during the downcutting of the Grayling Creek Valley. This creek, 

 following the well-known habit of streams, formerly cut its channel 

 westward along the strike of the northward-dipping sedimentary rocks, 

 gradually deepening its channel until, reaching the underlying crystalline 

 schists, it was compelled to continue in the same course, deepening and 

 widening the valley, whose straight northerly walls are due to the absence 

 of lateral drainage channels consequent upon the northward dip of the 

 strata. The same configuration is seen in the upper valley of the Gallatin 

 River and the valley of Fawn Creek, where the mountain gorges are cut 

 in sedimentary rocks. 



The topography of this vicinity is so closely dependent on the charac- 

 ter and position of the strata immediately overlying the gneiss, and these 

 strata have been tilted, curved, and faulted to such an extent, that it is 

 advisable to postpone the description of this area until the less distributed 

 rocks lying immediately east of the main body of crystalline schists have 

 been described. 



The region just mentioned lies east of the main gneissic area, and is 



