178 DESCRIPTIVE GEOLOGY, 



also yellowish undichroitic needles, which Professor Zirkel thinks may be 

 goethite. 



Along the western flanks, the basalt has a coarsely vesicular structure, • 

 the round cavities being filled with white opaque carbonate of lime, hav- 

 ing something of a concretionary structure. On the surface of the mesa, to 

 the west of the mountain, is a greenish-gray tufa, looking like a palagonite- 

 tufa, enclosing angular fragments of this vesicular basalt. It is a granular 

 mass, made up, as shown by microscopical examination, of fragments of 

 yellowish glass, augite crystals, magnetite grains, rounded quartz with fluid- 

 inclusions, and the black needles characteristic of crystalline slates, together 

 with decomposed orthoclase, in a calcareous matrix. It would seem, there- 

 fore, to be composed of the debris, partly of the basalts and partly of the 

 neighboring Archsean rocks, wdiich, however, have not been observed at the 

 surface within 8 miles of this occurrence. 



Along the benches above the south bank of the Little Snake River, 

 below the mouth of Slater's Fork, are other little basalt knolls, which prob- 

 ably belong to the outflow of Navesink Peak. This is the only important 

 basaltic peak which has a conical shape, and, although its altitude is consid- 

 erably less than that of the adjoining mountain-mass of Mount Weltha, it 

 forms a more prominent landmark when seen from the Tertiary plains of 

 the Green River Basin. High up on its northern face are remnants of beds 

 of fine white sands, whose ddbris fill the ravines leading northward. 



The basalt of Navesink Peak is a dark- gray, finely crystalline rock, 

 having the texture of an anamesite. Black augites and yellow olivines are 

 the only crystals that can be detected macroscopically, but the microscope 

 shows also magnetite, biotite, nepheline, and a little triclinic feldspar, while 

 the augites and olivines abound in glass-inclusions. 



An analysis of this nepheline-basalt was made by Mr. R. W. Wood- 

 ward, with the following result : 



Silica 48.60 48.46 



Alumina 15.78 15.61 . 



Ferrous oxide 10.11 . 10.31 



Lime 8.34 8.33 



