656 
ing the older and more artificial ones, and 
that there is ample hope for devising phys- 
ical means of correlation that are more in 
harmony with the real nature of the prob- 
lems involved. 
Still more recently it has been suggested 
that the real basis of geological correlation 
should be found in the causes giving rise to 
and governing sedimentation. This in- 
volves primarily a classification founded 
upon mountain-making movements. It is 
proposed, therefore, to emphasize this factor 
as fundamental in the marking off of the 
leading subdivisions of geological time, and 
to define general stratigraphical succession 
in accordance with the cycles of orogenic 
development, calling the classification or 
fundamental principle of correlation a sys- 
tematic arrangement by mountains, or oro- 
taxis. It is believed to overcome many of 
the difficulties that are usually encountered 
in correlation, in that it enables successions 
of strata to be paralleled not only in proy- 
inces whose geological history has been 
similar, but in those in which it has been 
very different. 
CHARLES R. KEryes. 
CURRENT NOTES ON PHYSIOGRAPHY. 
THE ROCKY MOUNTAIN FRONT. 
Monocrarn XXVII., U.S. G.S., on the 
Geology of the Denver Basin in Colorado, 
by Emmons, Eldredge and Cross, gives 
some account of physiographic features 
amid its detailed description of geological 
structures. The mountain front, when 
seen from a distance, rises as if from the 
sea in a continuous but rugged slope. A 
closer examination shows that, after reach- 
ing a certain height, the frontal spurs as- 
cend more gradually to the main crest of 
the range, which lies unexpectedly far back. 
The upper valleys are wide open, but 
descend into deep and narrow gorges to- 
wards the ‘shore line.’ This is taken to 
indicate a revival of stream action by up- 
SCIENCE. 
[N. 8. Von. VI. No. 148. 
lift, after an advanced stage of denudation 
had been reached when the region stood at 
a lower level. The hog-back ridge of the 
upturned foot-hill strata (Dakota), with 
the longitudinal subsequent valleys behind 
it, is generally even and continuous. 
Zigzag turns are formed on occasional 
folds; lapses of the ridge are noted at Gol- 
den and Boulder, where the Dakota sand- 
stone was not deposited. The moderate 
relief of the Plains is shown to result from 
extensive denudation ; the uppermost mem- 
bers of the series are broadly stripped off, 
leaving wide valleys between uplands and 
mesas of significant relief to the cross coun- 
try traveller, but broadly plain in compari- 
son to the bold mountain front. 
CASTLE MOUNTAIN, MONTANA. 
CastLEe Mountain is an outlying member 
of the Front range of the Rocky mountains 
in Montana, between the Missouri and Yel- 
lowstone rivers. It is described by Weed 
and Pirsson (Bull. 139, U.8.G.8.) as a 
“ dissected voleano,’ although the considera- 
ble cone that must have once risen here (as 
attested by lava flows and tuffs) has been 
almost completely denuded, and the exist- 
ing mountain of massive granite is hypo- 
thetically represented as of laccolitic struc- 
ture in the corrugated beds of the heavy 
stratified series; the granites being older 
than the effusive materials. As is so 
generally the case in the Rocky mountains, 
moderate deformation occurred after exten- 
sive denudation of the corrugated strata, 
aud considerable stratified deposits now 
mark the sites of lakes thus formed; the 
plains of Lake Smith, west of Castle Moun- 
tain, being the local illustration of this geo- 
graphical element. The volcanic eruptions 
were antecedent to the lacustrine period, 
as the lake beds consist largely of stratified 
voleanic tuffs. Glacial action is indicated 
by well marked moraines which are referred 
to two epochs. 
