598 REPORT UNITED STATES GEOLOGICAL SURVEY. 



which continued after the flow. In the volcanic action of this period 

 perhaps a portion of the cause of the extinction of the lakes is to be 

 found. We find no lake deposits resting on the latest flow, and in the 

 lower Portneuf Valley we have already seen that a period of subserial 

 erosion intervened between the draining of the lake and the last outflow 

 of basalt. 



BEAR RIVER RANGE. 



West of Bear Lake Valley, separating it from Cache Valley and Gen- 

 tile Valley, and extending north to the bend of Bear Biver, is the Bear 

 Biver Bange. It terminates in a broad, round summit known as Sheep 

 Bock. This northern portion is very narrow and has a trend of about 

 north 20° west. South of this the range becomes broader, although it 

 decreases somewhat in elevation to North Pass. Still farther south the 

 structure becomes somewhat complex, and the entire width of the range 

 is about 16 or 17 miles. The waters of Logan Fork flow southward in 

 the central portions of the mountains. The best defined range here is 

 on the west, a succession of sharp peaks extending northward from 

 Logan Canon. This range or sub-range presents a steep and rugged front 

 towards Cache Valley. On the east side the mountains are plateau-like, 

 and this is the general character towards the north. The southern por- 

 tion of the range is drained almost entirely by Logan Fork, which oc- 

 cupies a broad valley, for the most part in Silurian or Pre-Silurian ? 

 rocks. Our examination was not sufficiently detailed to separate the 

 beds so as to recognize the various divisions, and on the map I have 

 been obliged to color all the beds below the Carboniferous as Silurian. 



Speaking of Logan Canon, Mr. Arnold Hague says:* "At the forks 

 of the canon, the beds rise more steeply, and a quartzite belt is seen 

 which may correspond to the Ogden quartzite. In the north fork of the 

 canon, fragments of granite indicate the probable existence of a granite 

 core to this ridge beyond the limits of the map to the north." These were 

 not seen by us, nor was the head of the north fork visited, but if granitic 

 rocks do show, the outcrop must be limited to the canon, for at the 

 points where we crossed .the range to the northward no granites were 

 seen. At the entrance to Logan Canon limestones outcrop dipping from 

 30° to 35° to the eastward. This gradually decreases until the beds 

 gradually become horizontal and then rise again with a western dip. 

 The following is a general section made as we hastily rode up the canon : 



Section Ao. 30. , 



1. Very dark blue limestones somewhat laminated, dip 35°. 



2. Bluish-gray limestones, dip 25°. 



3. Rather massive blue limestones, dip 20°. 



4. Shaly and laminated limestones, dip 15°. 



5. Light blue massive limestones, dip 10°. 



The total thickness of these limestones must be about 4,000 feet. 

 From about the middle of the caiion the following fossils were obtained: 



Productus % 



Rhynehonella pustulosa. 

 Rhynclwnetta rocTcymontanus. 



JEuonvphalus ? 



Spirifer striatus. 

 . Spirifer roclcymontanus. 



Spirifer (Martinia) ? 



Athyris (Spirigera) subtilita? 



* U. S. Geol. Exploration of the 40th ParaUel, Vol. II, page 407. 



