372 REPORT UNITED STATES GEOLOGICAL SURVEY. 



ical position inferior to the limestone in Station XX. Descending the 

 spur westerly into the pretty little valley of a tributary of the middle 

 fork of Willow or Porcupine Creek, perhaps half a mile from the above- 

 mentioned height, a heavy ledge of drab-gray limestone forms a vertical 

 dike, or hog-back, extending in an east-west direction diagonally across 

 the ridge. It is followed by a similar though lower ledge, inclined steeply 

 southward; as also does the first mentioned in places. These ledges 

 are apparently Jurassic, and probably belong to the Station XX uplift. 

 In the low, grassy, undulating slopes bordering the little valley of 

 Porcupine Creek, which is hemmed in on the south by the high volcanic- 

 capped bench two or three mdes west of Station XX, the surface shows 

 frequent, though obscure, exposures of red shales and gray sandstones. 

 But to the west, these soon pass under the volcanic flows which slope 

 down into the general level of the volcanic upland in which Willow 

 Creek is deeply cauoned. A similar but more isolated volcanic bench 

 borders this little valley on the west, and which has from this point of 

 view a butte-like appearance ; but on gaming the summit (7,200 feet), 

 it is found to be a plateau, sloping westward, and protected by a heavy 

 covering of drab and pinkish trachyte, and yellow laminated readily de- 

 composing varieties with, mica flakes, which splits into thin uneven 

 slabs, giving to the rock a bedded appearance. It clearly rests upon 

 the denuded edges of the tilted sedimentaries, but no trace of the pre- 

 viously mentioned vesicular obsidian lava was here observed. This butte- 

 like plateau is about four miles due west of Station XX, and little more 

 than a mile southwestward of Station XXI, from which it is separated 

 by the sag of Three-Deer Creek, which flows into the Porcupine; the 

 southwest side marking the limits of the continuous volcanic formation, 

 while to the northeast and east the hills are mainly sedimentary. A 

 section carried from this point along a northeasterly line, passing through 

 Station XXI, is given below : 



Section through Station XXI. 



1. Trachytic cap of plateau between Porcupine and Three-Deer Creeks. 



2. Eed shales and soft gray sandstones. 



3. Soft, false-bedded, shaly, gray sandstone, 15 feet exposed ; dip 

 50, X. 45° E. This ledge is partially concealed high in the northeast 

 face of the volcanic-capped plateau, but in the lower ground to the south- 

 east it is traced as a low hog-back ridge extending down into the Por- 

 cupine Yalley. 



4. Eed shales, 200 yards across the exposure. 



5. Coarse, crumbling, rather even-bedded gray sandstone, 5 feet ex- 

 posed, in low wall in the northeast flank of the volcanic plateau. Dip 

 57°, X. 40° E. 



6. Obscure exposures of blue and red shales and gray indurated sandy 

 beds, with calcite, 300 yards. 



7. Soft gray sandstones with reddish shales, terminating below in — 



8. Heavy ledge of soft, shaly, gray sandstone, 10 to 20 feet exposed. 



9. Slope to Three-Deer Creek, showing obscure exposures of soft gray 

 sandstone, with calcite fragments in the soil, 400 yards. 



10. Gray, thin-bedded sandstone, in low bluff 50 yards northeast of 

 the stream-bed. 



11. De'&m-eovered slope, 75 yards. 



12. Coarse, thin-bedded, gray sandstone, 10 feet exposed; dip 55°, 

 X. 45° E. 



