VOLCANIC ROCKS NORTH OF BERRY CREEK. 157 



the northern slopes are mantled by this material. Many of the bare areas 

 seen on these slopes show no rock in place, but are due to the washing out 

 of this soft drift material. The hig'hest strata seen consist of shale and 

 limestone, exposed back of the rhyolite hill west of the forks of Glade 

 Creek. 



No Laramie rocks were identified, but it is probable that the friable 

 sandstones carrying- a 4-foot seam of coal, exposed on the east bank of the 

 Snake below the mouth of Glade Creek, are of this age. The strata at 

 this locality strike E.-W. and dip 35° N., but on the slope above, at 7,400 

 feet, the beds strike N. 40° E. and dip 25° NW. 



volcanic rocks. — The erosion of the sedimentary rocks was unquestionablv 

 very great before the extravasation of volcanic material, for basic andesitic 

 tuff-breccia is found in the bottom of the valley of Berry Creek, resting upon 

 a very irregular surface of crystalline schist and Paleozoic strata. In places 

 it contains rounded pebbles of gneiss, with some of andesite. It probably 

 occupies what Avas an ancient valley draining northward, for on the east and 

 west of it sedimentary rocks rise to peaks a thousand and more feet higher 

 than the present lowest exposure of bi-eccia. The breccia forms a group of 

 small hills and knolls, rising 500 feet above the bottom of the valley. It is 

 overlain by massive rhyolitic lava, which also covered a very uneven sur- 

 face, being at present 600 to 800 feet thick above the breccia and 1,500 feet 

 thick just east of it, where it seems to have filled a depression between the 

 hill of breccia and the eastern sedimentary ridge. Toward the west it 

 thins out over the limestones at the base of Survey Peak, where it is 

 glassy and porphyritic, probably the bottom part of the sheet which, 

 farther east, is lithoidal and thinly laminated, somewhat resembling schist. 

 In this part of the rhyolite the lamination which corresponds to the planes 

 of flow in the lava dip to the north, indicating that the lava had been 

 moving over a northward-sloping surface. 



REGION WEST OF THE CRYSTALLINE AXIS. 



West of the fault that bounds the western side of the body of crystal- 

 line schists the sedimentary rocks have been upturned at a high angle, 

 causing them to dip steeply at about 80° W. for a short distance, beyond 

 which they become flatter. This structure is seen in the mountain south 

 of Owl Creek and in Forellen Peak and Survey Peak. In the western 



