184 DESCEIPTIVE GEOLOGY. 



the limits of the map, the river cuts a narrow canon in a body of basalt which 

 has broken throug'h the Archaean schists. The main mass is a dark, compact, 

 fine-grained basalt, rich in brown crystals of olivine. It is apparently a feld- 

 spar-basalt, but so fine-grained that but tew feldspar crystals can be distin- 

 guished, and no other individualized minerals. It contains frequent rounded 

 cavities, which are lined with white crystalline carbonate of lime. Over the 

 main body of the basalt is a flow of basaltic mud, or pumice, of a deep red 

 color, carrying fragments of the solid lava in a vesicular, porous matrix, of 

 which the vesicules are frequently filled by white carbonate of lime. Higher 

 up the stream, bej^ond the limits of the map, are considerable flows of basalt; 

 and a high table-land to the south, at the head of White River, which rises 

 to a height of nearly 10,000 feet, is evidently capped by flows of basalt. 

 The Archcean rocks are exposed in some of the hills in the middle of the 

 valley, having apparently the same strike and dip with the main body, 

 though much obscured by surface accumulations. The low, flat ridge to 

 the west presents abrupt escarpments toward this valley, along the foot of 

 which are occasional exposures of thin, red sandstones, overlaid by siliceous 

 limestones, while the summit of the ridge is covered by the white and buff 

 sandstones which are seen at the gap. These limestones and red sandstones 

 evidently represent the Triassic and Jurassic at this point. Their thickness 

 cannot be estimated, but would seem to be very much less than that 

 observed to the north and west. On the low divide between the western 

 branch of Moore's Fork and the valley next west, at the extreme southern 

 limit of the map, are exposed the same siliceous limestones, here having a 

 silky semi-crystalline texture, and containing imperfect casts of fossils ; 

 they dip at very high angles, and are succeeded to the westward by white 

 sandstones somewhat metamorphosed, and the blue clays of the Colorado 

 Cretaceous. The most easterly of these limestone exposures has a westerly 

 dip, while farther west the next observed dip was eastward, and there would 

 seem to be here either a sharp synclinal, or, what is more probable, an in- 

 verted dip in the beds. The exposures are not sufficiently continuous to 

 afford a satisfactory section. The strike of the beds is approximately north 

 at this point, but bends to the eastward in the ridge farther north, where the 

 dip also shallows out. In the broad, narrow valley to the west of this ridge, 



