210 GEOLOGY OF THE BLACK HILLS. 



occasion to advert, that the rivers flow directly across the mountain instead 

 of going around it. 



In speaking of the elongated dome of strata which we have recon- 

 structed in the imagination, we have been careful to make use of the adjec- 

 tive "ideal," and to this we have given a double meaning. The dome is 

 ideal because we have constructed it as a logical deduction from a mass of 

 observed facts, but it is also ideal because it never had a real existence. 

 We have excellent reason to believe that when the uplift began the site of 

 the Hills was either dry land or the bed of a shallow lake, so that the rising 

 arch was subject to the attack of the elements either at its inception or 

 immediately after. The higher it rose the greater was its exposure and the 

 more rapidly was its summit worn away. When it reached the full 

 measure of its magnitude as an orographic protuberance, its destruction by 

 elemental warfare had progressed so far that the magnificence of our ideal 

 structure was very far from realization. Of the slowness of the uprising 

 of the mountain we have no evidence in what we were able to glean during 

 our hurried study. For aught that we learn from local observations the 

 mountains may have shot up with the speed of a growing vine, but in the 

 growth of other mountains there are many things to show that their pro- 

 gress was secular, and upon no plausible theory of the origin of mountains 

 can it be supposed that their birth-labor is other than exceedingly slow. 

 The earthquake is but the passing pang that records a unit of progress; it 

 is only by the combination of many such units, separated often by wide 

 intervals of time, that the great result is accomplished. 



2. — The relation of the topography to the structure. 



There are two conditions which more than any other determine the 

 forms of mountains. The first and most important is the rock structure. 

 The second is the drainage system, or the arrangement of the lines of drainage. 

 The rock structure always determines the type of the resultant form, and 

 usually it is largely concerned in the determination of the drainage system. 



The rock structure of a mountain is the arrangement of the various 

 masses which compose it. When those masses are chiefly sedimentary 

 rocks, as in the present case, the structure is readily divisible into two 



