644 KEPOET UNITED STATES GEOLOGICAL SURVEY. 



points, where ir van over small inequalities <>f llu- surface beneath, il now stands in 

 low mounds, which would not have been the case if it had been very fluid. That 

 these mounds were no1 all formed by an undermining and sinking of the surrounding 

 mass, to which some of them have very properly been referred, is proved by the taper- 

 ing shape of the closely-fitting blocks which form the arch. 



Although I did find two layers of basalt in the upper part of the 

 canon separated, I am still inclined to think that the source of the ba- 

 salt in the lower valley of the Portneuf was somewhere in the Snake 

 Eiver plain. Dr. Hayden was also of the opinion that the Snake River 

 plain was a centre. 



The source of the basalt that separates the Portneuf Eiver and Marsh 

 Greek is undoubtedly the craters in the Blackfoot Basin and Basalt 

 Valley, as the connection is easily traced through the Portneuf Gallon. 

 The craters that are still standing in the Blackfoot and Bear River re- 

 gion are very perfect. These basaltic flows extend into Mr. St. John's 

 district, and the reader is referred to his report for details in regard to 

 its occurrence there. 



