96 GEOLOGICAL SURVEY OF THE TERRITORIES. 



trachyte capping it as we go toward the hills. On the south side the 

 rocks are gneissic, as they also are in the canon. At the upper end of 

 the valley, oi^ the north side, is an outcrop of soft yellowish gray sand- 

 stone. Farther down the breccia, instead of resting on this sandstone, 

 appears to be directly upon the schists. At station 71, however, between 

 the schists and the breccia there is a belt of sandstone that seems to 

 have been metamorphosed.' Above the breccia there is a capping of 

 trachyte. 



The canon is only about a mile and a halWn length, and in it the river 

 has rather a tortuous course, flowing at first south of west and afterwards 

 north of west. The walls are not very high, never exceeding 300 feet. 

 Below, the river is in another valley similar to the one above. This, 

 however, is not as large, being only two miles long and a mile wide iu 

 the widest portion. It is not so well grassed as the valley above. The 

 schists still compose the hills on the lower side, while on the north there 

 are sandstones, breccia, and trachyte in long mesalike ridges that 

 extend from the margin of the valley toward the hills in which station 

 31 is situated. 



Leaving this valley the river again goes into canon and keeps so 

 almost all the way to the mouth of the Lake Fork, broken only by small 

 and unimportant valleys. The rocks in which the river-channel lies are 

 schists, and the walls never exceed about one hundred and fifty feet 

 in height. This granitic rock forms a narrow strip, leaving a bench or 

 shelf on both sides of the river, reaching back to the edge of the bluffs. 



On top of the schists, forming the base of the bluif^ are the sand- 

 stones that probably belong to the Dakota group. As we proceed down 

 the river we find on top of these, Jblack and yellow shales, above which 

 is the breccia to which I have so frequently referred in this chapter. 

 This, in a great many places, has weathered into sharp pinnacles and 

 towers. It is capped with obsidian and trachyte. 



A section of thebluffs will be given in another place, when the differ- 

 ent layers will be particularly described. 



These bluffs are on both sides of the river, and form the edge of the 

 mesas that are so characteristic of this part of the valley of the Gun- 

 nison, extending back from the river on both sides. 



Below station 71 the top of the mesa is about 500 feet above the 

 river-level, and at station 73 it has increased to 1,230 feet, the general 

 surface really remaining at the same level on both sides, while the 

 river in its progress has cut deeper and deeper, making the bluffs nearly 

 twice as high. This height is still greater farther below, as we will find 

 when we speak of the caiion. 



The streams joining the Gunnison cut deeply into the surface, divid- 

 ing the original mesa into many others. The canons thus formed have 

 almost perpendicular walls. There appeared to be more than one 

 layer of the trachyte, for, from the edge of the mesa a higher outcrop 

 can be seen. I was notable this year to determine definitely the relations 

 between the trachytic flows and the breccia on the mountainous mass 

 around station 31. Another year I hope to explain it. 



Above the mouth of the Lake Fork, the Gunnison flows through a 

 small open valley covered with grass and sage-brush, in which it is 

 joined by a branch of some size from the north. From this valley the 

 river plunges into the largest caiion in its entire course. Lake Fork is 

 itself in a deep canon, cut through dark, micaceous schists, and until 

 one comes to the edge of the gorge, he has no idea of its size or extent. 

 Gunnison's wagon-trail is obliged to cross it along way back from the 

 river, and come down the stream on the opposite side. 



