Postelsta 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 XX XI); 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, Tully 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 rhythm of sedi- 
