IP s t e I s 1 a 323 



the upturned edges of one continuous formation. 

 Here and there faults are to be seen where the 

 rocks have slipped, and on one side or the other 

 of this structural break the rock stands relatively 

 higher or lower than before (Plate XXXI); in 

 places it stands at a higher angle to the surface of 

 the sea than the average: occasionally the sea has 

 worn away the rocks more nearly parallel with the 

 strike than is the rule; but, when all conditions 

 and accidents are given their full value of quali- 

 fication, the total vertical thickness cannot be 

 less than 12,000 to 15,000 feet, fully two and 

 one half miles of sands and clays, before a 

 change in physical conditions brought cessation 

 of deposit. An interval of erosion and alter- 

 ation was inaugurated. 



And thus the question of time is reached. 

 While the geologist must look in vain for a 

 measuring rod upon this stretch of ancient 

 coast, one has been found in the Cretaceous 

 along the base of the Rocky mountains where 

 the Benton, Niobara and Pierre groups have a 

 total thickness of 3,900 feet. Grove K. Gil- 

 bert has associated an obscure rhvthm of sedi- 



