644 KEPOET UXITED STATES GEOLOGICAL SUEVEY. 



points, where it ran over small inequalities of the surface heneath, it now stands in 

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

 tl'.ese mounds Avere not all formed by an undermining and sinking of the surrounding 

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

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



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

 cafion sej^arated, I am still inclined to think that the source of the ba- 

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

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

 plain was a centre. 



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

 Creek is undoubtedly the craters in the Blackfoot Basin and Basalt 

 Yalley, as the connection is easily traced through the Portneuf Canon. 

 The craters that are still standing in the Blackfoot and Bear Eiver 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. 



