INFILLING OF THE VALLEYS 289 



to them. In this state the channels proceeded to fill and the detrital 

 slopes to climb higher upon the sides of the mountains. Wherever by 

 the erosive process a part of the lower slopes of the ranges came to have 

 a gentle declivity the accumulating debris would mantle it over; where 

 outlying peaks became islanded in the detrital deposits they continued 

 to wear down until, if their original height were not too great, the slowly 

 rising surface would overtop them. 



It is evident from the sections seen at Butte and elsewhere that the 

 mantle of detritus lying on the bed rock tends to preserve that rock from 

 further decay. This is brought about by a simple process, one com- 

 monly occurring in all gravels made up of rocks that afford soluble 

 material, such as iron or lime. This dissolved matter finds its way 

 down upon the bed rock, and there cements the lower part of the de- 

 tritus into a hard mass which is impervious to water. Whenever we 

 attain this contact of the detritus with the crystalline rocks we find this 

 protecting layer, with the result that decay does not seem readily to occur 

 beneath it. It is otherwise with the materials of the valley deposits be- 

 tween the upper section, which is prevailingly very dry, and the cemented 

 layer. In this zone the process of decomposition appears to be tolerably 

 rapid, as is shown by the considerable discharge in the seeping springs 

 that appear along the streams when the water is commonly much charged 

 with lime and iron. 



In the manner above noted we can perhaps account for the peculiar 

 shoulders of the bed-rock valleys which, apparently in a somewhat gen- 

 eral way, extend from the margin of the steep gorges to the foot of the 

 exposed portion of the mountain ranges. The process of occlusion of the 

 narrow steep-sided central part of the valley probably would go forward 

 with such rapidity that the exposed cliffs would not be worn down so 

 that they could become covered. As the deposit rose higher in the trough 

 it would encounter gradual slopes on which the detrital coating would, 

 even where thin, preserve the bed rocks from further decay. It is evi- 

 dent that with a slope of about 150 feet in the mile the materials of the 

 benches are commonly able to keep their place so that any declivities of 

 less inclination would be protected. In this way, as the erosion forced 

 back the faces of the mountains the valley would be widened, the gain 

 representing the advance in the decay of the mountain fronts down to 

 the level where the detrital accumulations began to protect the rocks. 



This view as to the way in which the broad valleys widen may help 

 us to understand the peculiarly straight walls which the ranges that 

 border them present. As before noted, the generally rectilinear aspect 

 of these mountain fronts demands explanation. It is, indeed, as strik- 

 ing an element as is the width of those vales. The feature may be due, 



