EXPANSION OF CONDENSED PHRASES . 101 



made in the second or third line, namely, "a highland of disordered rocks, 

 which shows signs of having been long ago worn down from its initially 

 greater mass to a surface of faint relief, slowly depressed and broadly 

 buried." Of what value, one may ask, is this reference to a long cycle of 

 ancient erosion and planation before the depression and burial of the 

 worn-down mass, and hence long before its monoclinal uplift and later 

 erosion ? In answer it may be said that the mention of ancient planation 

 of the crystallines before the covering strata were laid down was inten- 

 tionally introduced, because it is a factor of prime geographical signifi- 

 cance in leading to an intelligent conception of one of the most charac- 

 teristic features of the Front Range, namely, its abrupt and almost 

 rectilinear border toward the plains, for if we imagine a compound mass, 

 of which the under member was a worn-do\vn body of disordered and 

 resistant crystallines, while the upper member was a heavy series of sedi- 

 mentary strata, and then conceive it to suffer monoclinal deformation, 

 peneplanation, renewed uplift, and renewed erosion, by which the weaker 

 strata east of the monocline are again peneplained, while the resistant 

 rocks of the mountains are only submaturely dissected, the mountain area 

 must then exhibit along its border a stripped part of the surface that was 

 long ago worn down to faint relief, and hence the mountainous highland 

 today must have an abrupt and rectilinear margin, in which the stripped 

 part of the surface of ancient j^lanation, tilted into a monoclinal slope, is 

 exposed between the higher peneplain of the hard-rock highlands and the 

 newer and lower peneplain of the weak-rock plains. The stripped plana- 

 tion surface of the mountain front will, of course, be cut down by numer- 

 ous revived streams, where their valleys open from the highland ; thus 

 the continuous mountain border is transformed into a series of trape- 

 zoidal or triangular facets, of which the upper part will be somewhat the 

 worse for wear, and the lower part may be still more or less covered by 

 the basal members of the tilted plains strata ; but for all that the moun- 

 tain front, thus explained, must be conceived as abrupt and essentially 

 rectilinear. The mountain front is indeed a great fragment of a fine 

 geographical fossil, in the sense of being part of an ancient surface, either 

 a land area or a sea floor, long preserved by depression and burial, and 

 now, like many smaller fossil, brought to light by uplift and erosion, its 

 higher extension completely destroyed and its exposed belt somewhat 

 weather-beaten, while its lowest, deep-lying part still remains buried. It 

 is a pleasure to put on record at this time and place that it was my friend 

 and classmate, the lamented Archibald E. Marvine, who, as a member of 

 Whitney's party in 1869 and of Hayden's Survey a few years later, first 

 recognized the origin and the significance of this ancient surface of 

 erosion. 



