426 DESCRIPTIVE GEOLOGY. 



terrace-lines, and presents everywhere marked evidence of having been sub- 

 merged within recent geological time. The greater part of this area is 

 more or less concealed beneath loose sand and basaltic detrital material of 

 the Upper Quaternary, Only here and there, the presence of the under- 

 lying basalt is indicated by little hills and knolls which rise above the plain. 



North of the Terrace Mountains, the basalts reach a somewhat higher 

 efevation, forming low, broad tables with isolated domes. Of these, Red 

 Dome, just north of the railroad, is the most prominent, presenting a con- 

 spicuous landmark in the region north of the lake. These basalts are char- 

 acterized by a dense, fine-grained texture, but are occasionally quite porous, 

 when they possess a reddish tinge, especially on the weathered surface, the 

 color giving the name to the dome already mentioned. Wherever the slope 

 of basalts was noted, it indicated a slight inclination to the southward, toward 

 the desert. In a number of localities, at the head of the lake, salt springs 

 occur on the desert, many of them quite cold and clear, all carrying con- 

 siderable common salt in solution ; many of them, in fact, being dense brines. 



Between Red Dome and the Raft River Mountains lies an irregular 

 mass of hills, which only in a few points attain an elevation of more than 

 1,000 feet above the desert-level. The greater part of these hills are below 

 the well-recognized level of the old lake, and, like the hills to the eastward, 

 everywhere show evidences of the recent occupation of the country by the 

 lake waters. They are made up of dark-gray limestone broken through 

 and partly concealed by heavy outbursts of basalt. Nothing can be said 

 definitely of the structure of the limestones of these hills, as they are much 

 disturbed by the Tertiary volcanic rocks, or else concealed by the still 

 later loose Quaternary gravel. No fossils were found which determined 

 accurately the horizons of these limestones, but there can be little doubt 

 that they belong to the upper members of the Wahsatch limestones, which 

 characterize all the mountain-ranges north of the lake. On the west side 

 of Dufi" Creek, basalts find their greatest development in broad, heavy, 

 tabular masses, lying inclined at an angle of about 2° to the southeast, and 

 presenting precipitous cliffs toward the creek. The rock is a compact black 

 basalt, with a crypto -crystalline groundmass and uniform texture. On the 

 basalt slopes west of Duff Creek, and on the south side facing the desert, 

 the terrace-benches, which are so characteristic of the entire region, are 



