3i8 Sierra Club Bulletin 



where the granite comes in contact with the slates, and 

 for a considerable depth beneath the line of contact, it par- 

 takes, in a greater or less degree, of the physical structure of 

 slates, enabling us to determine the fact that in many places 

 slates have covered the granite where none are now visible for 

 miles, and also furnishing data by which to approximate the 

 depth at which these surfaces lie beneath the original summit 

 of the granite. Phenomena relating to this portion of the ar- 

 gument abound in the upper basins of the tributary streams of 

 the Tuolumne and Merced ; for their presentation, however, in 

 detail, we have no space in these brief outlines. 



If, therefore, we would restore this section of the range to 

 its unglaciated condition, we would have, first, to fill up all the 

 valleys and canons. Secondly, all the granite domes and peaks 

 would have to be buried until the surface reached the level of 

 the line of contact with the slates. Thirdly, in the yet grander 

 restoration of the missing portions of both granite and slates 

 up the line between the summit slates and those of the base, as 

 indicated in Fig. lo by the dotted line, the maximum thickness 

 of the restored rocks in the middle region would not be less 

 than a mile and a half, and average a mile. But, because the 

 summit peaks are only sharp residual fragments, and the foot- 

 hills rounded residual fragments, when all the intervening re- 

 gion is restored up to the dotted line in the figure, we still have 

 only partially reconstructed the range, for the summits may 

 have towered many thousands of feet above their present 

 heights. And when we consider that residual glaciers are still 

 engaged in lowering the summits which are already worn to 

 mere blades and pinnacles, it will not seem improbable that the 

 whole quantity of glacial denudation in the middle region of 

 the western flank of the Sierra considerably exceeds a mile in 

 average depth. So great was the amount of chipping required 

 to bring out the present architecture of the Sierra. 



Reprinted from the Overland Monthly of August, 1874. This is the fourth of 

 a series of seven studies in which Mr. Muir developed his theories of the geology 

 of the Sierra. — Editor. 



