186 DESCRIPTIVE GEOLOGY. 



coal seams characteristic of that horizon in this region were found. To the 

 north of the river, these beds rise, approximately with the slope of the 

 valley, toward the Elkhead Mountains, while in descending the river below 

 the ford a series of gentle folds is crossed, whose axis is approximately north 

 and south, forming slight undulations in the almost horizontal beds. The 

 only fossils obtained from these upper beds were a few uncharacteristic 

 species of Ostrea. 



On the south bank of the river, between the ford and the mouth of 

 Fortification Creek, is exposed a small body of nepheline-basalt, which 

 resembles mineralogically the basalts of the Elkhead Mountains. In its 

 physical habit, hoAvever, it is a more compact, close-grained rock, not at all 

 vesicular, showing yellowish-brown crystals of olivine and black augites in 

 a dark-gray, homogeneous groundmass. The summit of the ridge, between 

 this point and the valley of Williams Fork, is covered with fragments and 

 rounded pebbles of basalt, showing the probable existence of considerable 

 basalt flows in the high ridge to the south, beyond the limits of the map, 

 which divides the Yampa from the White Eiver. 



Fortification Peak is a little remnant of a basalt-flow from the Elkhead 

 Mountains, which has protected its base of white sandstone from erosion, and 

 stands out boldly above the low rolling country in a castellated hill, which 

 rises some 400 to 600 feet above the general level. The beds of basalt 

 which form its summit are only from 50 to 100 feet in thickness, and have 

 preserved the general shape of the sandstone ridges which they covered, 

 showing a remnant of a valley on the very summit of the peak. The basalt 

 is a very jporous and scoriaceous rock, and on its upper weathered surface 

 is often colored a bright red, and has the ropy texture of a recent lava flow. 

 In fresh fracture, it shows a dark, vesicular mass, in which, however, but 

 few crystalline ingredients are visible to the naked eye. The vesicules have 

 frequently a thin, blue, interior coating, and are sometimes filled with white 

 carbonate of hme. Under the microscope, the rock is rather coarse-grained 

 and rich in augite; it contains also oh vine, magnetite, and some triclinic 

 feldspar ; a part of the colorless groundmass is proved to be nepheline. When 

 treated with acid, the solution gives a precipitate of gelatinous silica, too 

 voluminous to be due to olivine alone. Both ohvines and augites contain 



