48 The American Geologist. July ' 1903 - 
worn gravel and bowlders identical with those of the modern 
stream bed, in which pot-holes are also forming. Farther up 
this valley we have similar and equally clear evidence of geo- 
logically recent faulting in the immense semi-detached block 
of Sherman mountain, rising precipitously from the north side 
of the valley between the village of Silver Plume and the group 
of rich mines, in the workings of which the fault fissures have 
been extensively and clearly developed and repeat the phenom- 
ena of the Aliunde Mines on the eastern slope of Leavenworth 
mountain. 
Taking a broader view of these phenomena, it appears im- 
probable that they are peculiar to this district, but rather that 
they are characteristic of many of the drainage systems of the 
mountains, especially towards the headwaters and the axis of 
the range. The idea is inevitably suggested that the elevation 
of this great mountain mass, the broadest and possibly one of 
the oldest of the Colorado ranges, has been recently, and may be 
still in progress ; and that while in the past the movement has 
been chiefly massive, developing the magnificent fault scarp 
overlooking the plains, it has in later times affected the axis 
more than the margin of the orographic block, leading to a 
marked tilting of the Cretaceous peneplain represented by the 
broad, grassy uplands or shoulders (alps) of the mountains: 
and in part, at least, the uplift is very locally differential and, 
in the Georgetown instance, in a way to accentuate the topo- 
graphy. 
EDITORIAL COMMENT. 
THE QUANTITATIVE CLASSIFICATION OF IGNEOUS ROCKS. 
With the introduction of the microscope into petrography 
it became early evident that existing systems of classification, 
if such they could be called, would need very decided modifi- 
cations to meet the new requirements. Indeed, the entire as- 
pect of matters changed so rapidly that, for a time, it seemed 
that confusion had been confounded, and a consistent and phil- 
osophical classification of rocks rendered hopeless. As time 
went on, however, as knowledge regarding structure, mag- 
inatic differentiation, and occurrences, increased, new schemes 
