654: GEOLOGY AND TOPOGRAPHY OF THE LEAD REGION. 



posited in the Lead region, the probabilities being rather to the con- 

 trary. Premising this, it follows, that the location of streams must 

 have depended on the natural configuration of the country, and the 

 superior advantages of certain strata in certain positions, predisposing 

 them to become the beds of streams. Other things being equal, sur- 

 face waters would naturally form a channel first in the more soft and 

 easily erosible strata lying along the line of strike of some soft for- 

 mation, and would cause a river to conform its first channel to its out- 

 cropping edge. 1 Simultaneously its tributaries would shape their 

 channels, approximately at right angles to the river, under the follow- 

 ing conditions: when the general slope and drainage of the country 

 is not contrary to the geological dip of the formations; which, in the 

 Lead region, does not appear ever to have been the case. The trib- 

 utaries on one side of the river thus formed would conform them- 

 selves to the natural dip of the underlying strata, sloping toward the 

 main river, and would be found wherever there were depressions, or 

 irregularities in the surface, suitable to their formation. These would 

 at their inception approximate to their final length and course, and 

 future changes in them would be confiaed to the 'deeper erosion of 

 their beds, and widening of their valleys; the formation of lateral 

 branches; the division of the head of the stream into several smaller 

 sources, and finally, the gradual recession of all the subordinate parts. 



With the tributaries on the other side of the principal river, a dif- 

 ferent order would prevail, as regards their position and growth. 

 They would at first be the merest rivulets, and increase only from ero- 

 sion ; and their beds would lie across the edges of the strata. There 

 would be only a very limited extent of country tributary to the river 

 on this side; the great volume of its water being derived from the 

 tributaries of the other side. The dividing ridge would thus be very 

 near the river, and a second set of long streams, tributary to some 

 other river, would here take their rise and flow away. 



In the process of time, the main river would slowly cut its way 

 through the soft formation, in which it had its original bed, into 

 and through those which underlaid it. This might at first be accom- 

 panied by a slight recession parallel to the line of strike; such a 

 movement, however, could not be of long duration, but would become 

 less as the valley became deeper; because any such recession would 

 necessitate the removal of all the overlying formations. Finally the 

 small streams flowing across the strata would cut their valley back 

 from the river; the dividing ridge would recede, and their sources 

 would, from the position of the strata, be in steep and precipitous ra- 

 1 See Report of Board o Rearents of the University of Minnesota, for 1879, pp. 46 and 47. 



