378 EEPORT UNITED STATES GEOLOGICAL SUEVEY. 



20. Coarse vesicular, rusty lava, occurring in abraded bowlder-like 

 masses iu the sloi)ing plateau-like summit at Station XXIII. 



The rock last mentioned above is probably the same as that previously 

 mentioned as occurring in the watershed summits in the vicinity of 

 Stations XX, XXI, and northward. Only here it is associated witb 

 an inferior fragmental deposit, possibly also of volcanic origin, the pres- 

 ence of which is of more than ordinary interest for the evidence it 

 furnishes of the character and relative position of a kind of volcanic 

 jfroducts not of infrequent occuri-ence in this region. 



It is apparent that the sedimentary deposits in the northwest flank of 

 XXIII are the same as those met with on the west slope of the range a 

 few miles distant, and it is not improbable that the limestones prove to 

 be Jurassic, though the few traces of organic remains, unfortunately, are 

 not in a condition suitable for identification. In the steej) south and 

 southeast slope, red shales show here and there, 100 to 200 feet beneath 

 the conglomerate, whose disintegration has strewn the declivity with 

 debris. In the foot of the declivity, perhaps a quarter of a mile south 

 of the summit, a ledge of drab, gray -mottled, rough weathered, spar- 

 seamed limestone apjiears, showing an exposed thickness of 8 feet, and 

 dipping 35°,X. 40° W. It contains afewfragments of Ostrea strigidecula (.^), 

 determining its Jurassic age. From the latter point the saddle ridge 

 extends southward to a high plateau-like summit, one and a half miles 

 distant, in the slightly tipped ui^ east and southeast rim of which there 

 appears a ledge of rusty vesicular lava, inclining gently northwestward 

 with the surface, which is covered with groves of fir antl aspen and open 

 grassy glades. Midway and to the south along the coniiecting spur, 

 heavy ledges of gray and light drab heavy bedded limestones, including 

 a bed of reddish, gray laminated sandstones, 5 feet exi)osed, and red 

 shales, lap up on the western flank, and at one point arch over the ridge 

 in an anticlinal fold, with dips IST. 35° to 55° E., at angles of 25° to 45° 

 on the one hand, and S. 40° W., at an angle of 25°, more or less, in the 

 oi)posite flank. The limestones of the west flank of the fold form a rocky 

 spur leading up to the summit of the plateau, where they pass from 

 sight beneath the volcanic flow. Bowlder-like masses of the latter, Avith 

 fragments of black obsidian, were found scattered along the crest of the 

 saddle-ridge ; but the volcanic conglomerate was not again detected in 

 the higli mountain summits. 



The sedimentary beds above alluded to are very variable in the direc- 

 tion of their strike, so much so as to greatly increase the difiiculty of 

 tracing the folds into which they have been ui>lifted for any distance iu 

 so broken a country as this. From what we have been able to learn, it 

 would appear that the strata, in this northern portion of the range, have 

 been subjected to disturbing agencies even more complicated in the re- 

 sult of tiieir action than what obtained in the range to the south. 



In the southeast flank of the little mountain plateau the sedimentary 

 ledges again appear, where, as seen from Station XXIV, they exhibit 

 an interesting abrupt fold, which has every ai)pearance of being inverted 

 on the northeast. The exposed strata involved in this overturn seem 

 to be identical with the exposures observed iu the before-mentioned fold 

 in the saddle-ridge on the opi^osite side of the plateau. These beds 

 again appear in the crest of the ridge just southwest of Station XXIV, 

 where, however, they are simply uplifted into a low arch of quite sub- 

 ordinate importance, forming merely a wrinkle iu the generally soutli- 

 westerly inclined strata. 



Half a mile or so southeast of the volcanic-capped plateau, witb which 



