118 E. C. ANDREWS. 



and to the summit of Mount Darwin (13,870 feet). The 

 descent thence was made of the great east fault front of 

 the Sierras. The base of this giant scarp was then skirted 

 to Mono Lake. The Sierras were here ascended at Bloody 

 Oanon. Mount Dana (13,000 feet) was visited, and the 

 Yosemite was approached by way of Tuolumne and Cloud's 

 Rest. Trips also to the Grand Canon of Arizona and to 

 the Wasatch Range of Utah were taken at Dr. Gilbert's 

 suggestion, and Pike's Peak and Cripple Creek areas were 

 also studied. To Dr. Gilbert, our master in physiographioal 

 science, the writer is under a peculiar debt of gratitude for 

 the trouble taken by him in pointing out all points of 

 interest in this wonderland, and for supplying an explana- 

 tion of the greater " facts of form" there seen. 



The general account of the wonderful topographic and 

 volcanic forms seen on that trip will doubtless be written 

 by Dr. Gilbert and Mr. Willard D. Johnson who have both 

 made a close study of them. 



As a result of that trip the writer wrote a paper on 

 "Corrasion by Gravity Streams." 1 But after the prepara- 

 tion of that paper it was seen that the application of the 

 principles therein deduced suggested the complete dis- 

 mantlement of one raised peneplain surface during the 

 development therein of another peneplain at a lower level, 

 if both such surfaces had been excavated in rocks compar- 

 able in hardness and resistance to the forces of erosion* 

 This leads to the main thesis of the present note, namely, 

 that in areas of homogeneous rocks or of rocks comparable 

 in hardness, such as dense sandstones, quartzites, granites 

 or crystalline schists, the existence of two peneplain sur- 

 faces in association but separated by youthful topography 

 must be explained by activities other than those of ordinary 



1 This Journal, Vol. xliv, 1910, p, 204. 



* A joint paper on the physiographic criteria of faulting iu Eastern 

 Australia is in preparation by Mr. C. A. Siissmilch and the writer. 



