INFLUENCE OF TOPOGRAPHY. 1221 



epochs of extensive base leveling from pre-Cambrian to Tertiary time, it is 

 seen that shifting of drainage lines in consequence of different cycles of 

 topographic erosion may be of great consequence. 



Doubtless in consequence of changes in drainage many ore deposits 

 which, when below valleys, received a first concentration by ascending 

 waters, are now well up on slopes or even at crests. A change of this 

 kind is especially favorable to the development of ore deposits which are 

 formed by two concentrations, the first by ascending and the second by 

 descending waters. In an early stage of the history of a deposit it may be 

 in a favorable place to receive a first contribution of ore. Later, if in con- 

 sequence of a topographic change, it be on higher ground, it is then in a 

 favorable place for the work of descending waters. Although it is difficult 

 to prove, I have little doubt that many ore deposits have had this history. 



Where a region has been base leveled it is very difficult to reconstruct 

 the topographic conditions which led to the concentration of the ores. To 

 illustrate, the Lake Superior copper deposits are believed to have been 

 deposited by ascending waters. The waters may have made their way 

 down along fault fractures and through the minor pores to the sloping con- 

 glomerate beds and have risen along the beds which had their surface out- 

 crop at a lower elevation than the feeding areas. But all this is purely 

 hypothetical, for the region has been base leveled and the copper deposits 

 unquestionably developed before the end of the base-leveling period. Since 

 that time the region has been uplifted and the present topography incised 

 in the base-leveled plateau. So far as one can see the present topography 

 has no recognizable relation to the ore deposits. 



From the foregoing' it is apparent that the relation of topography to 

 ore deposits is an important one, and also that in many districts its deci- 

 pherment is a difficult problem. 



PHYSICAL REVOLUTIONS. 



The genesis of many ore deposits is undoubtedly further complicated 

 by physical revolutions of various kinds. After an ore deposit has partly 

 formed, either by ascending or descending waters, or both, the region may 

 go through a physical revolution, and after the revolution the concentration 

 of the ores may again be taken up by nature's processes. 



