shale was laid hare, and so soon as this occurred the con- 
ditions for lateral corrasion were complete. With trachyte 
in the peaks and shale upon the slopes planation would 
naturally result, and a drainage system would be arranged 
i 
about the dikes as a center without regard to the curves of 
the strata. The subsequent removal of the shale would im- 
part its drainage to the underlying sandstones. 
"Either hypothesis is competent to explain the facts, but 
the data do not warrant the adoption of one to the exclusion 
of the other. The waterways of the Leaser arch may be either 
antecedent, or superimposed by planation. The Greater arch 
may have been the first to rise or the last." 
Prom Report of the Geology of the 
Henry Mountains, by G. K. Gilbert. 
Dept, of the Interior,. U. S. Geographi- 
cal and Geological Survey of the Rocky 
Mountain Region. 1877. 
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