222 GEOLOGY OF THE BLACK HILLS. 



Salt water seems to have covered the surface continually to the close of the 

 Mesozoic, and it was probably succeeded without interval by the fresh water 

 of a Tertiary lake. 



Before the water retired the uprising of the Hills began, and as soon 

 as the arch of strata appeared above the surface an erosion commenced 

 which has not yet ceased. Afterward a second Tertiary lake laved the 

 flanks of the uplift, half buried its foot with sediments, and then disap- 

 peared. Upon its dry bed the courses of the modern rivers were traced. 

 The streams that had been transporting detritus from the Hills and casting it 

 into the lake now found their way to the rivers and transferred their bur- 

 dens to them. Little by little, but continuously, they have eaten away the 

 substance of the Hills ever since. The canons they have cut, and which 

 appeal to our eyes as marvelous monuments of their industry, are the 

 least of their results. Since their labor began they have demolished and 

 removed one half of the entire mass of the uplift. Modest and feeble as 

 they seem, it is their ambition patiently to toil on until no vestige of the 

 Hills remains. 



