ROCK MATERIAL. 25 
jtill. Rattlesnake Hill in Stoekbridge, etc.) or ridges (Pine Hill). 
Although the ravines of the Green Mountain Range penetrate its 
lantling quartzite, neither that range nor the Rensselaer Plateau is 
I) intricately dissected as the schist masses of Group II. The soluble 
bcks (Group III) are confined almost entirely to the valleys, but on 
Sreen Peak and Mount Equinox the limestone reaches above the 
000-foot level, and the schist capping has in both mountains been 
lartially removed. Exceptionally, on the Green Mountain Range, 
lear the Addison and Rutland County line, a dolomite, which forms 
lie base of the great limestone formation of the Vermont Valley, cov- 
rs several square miles and forms a mountain 2,547 feet in' height. 
The region of Group IV, to the west, generally ranges between the 
100- and 800-foot levels. As the series of hills stretching along 1 it for 
Ivor 50 miles consists largely of the same material as the region itself, 
jhe origin of these hills must be attributed to their structure rather 
Mian to their material. 
This classification of rocks in reference to their erodibility and the 
Relations traced between them and the topography suggest a discus- 
lion of the question as to what in each case was the controlling factor 
In the development of the topography. It should be borne in mind 
Ihat the problem is not a simple one, for the rocks of each group 
>ften have associated with them other rocks of greater or less erodi- 
Ipility. Thus both the slate and the schist include some beds of 
fjjuartzite, and the quartzite some beds of schist. Limestone and dolo- 
mite are not equally soluble, and both are frequently quartzose or 
Inicaceous. In the next place, where a rock purely typical of one or 
[mother of these four groups does occur, its behavior under erosion 
may be greatly modified by peculiar physical properties. Thus a cliff 
bf thick-bedded, vitreous quartzite traversed by several systems of 
Joints is more easily disintegrated by frost than a thin-bedded, less 
ptreous, and less jointed one, and might prove even more erodible 
man a cliff of mica-schist. The determinant here is the vitreousness 
pf the rock, for to it the much jointing is due, the stress in both cases 
having been assumed as equal. This is also well illustrated by the 
fese of differential erosion cited on page 17. The controlling factor 
in the erodibility of the two rocks was the peculiar physical constitu- 
tion of the eruptive, probably its vitreousness, which caused it to 
scale in large conchoidal pieces, so that erosion made much more 
headAvay on the hard rock than it did on the soft one. Otherwise the 
dense crystalline mass of feldspar, hornblende, augite, and magnetite 
of the camptonite would have resisted erosion much more effectually 
than the microscopically matted sericite of the finely cleft slate. The 
a Not only the stress, but also its rapidity of application and the load, are assumed to 
he equal, for, given sufficient time under sufficient load, stress would have caused the fold- 
ing of both*rocks. 
