26 TACONIC PHYSIOGRAPHY. 
erodibility of any given rock is also largely determined by its posi j 
tion and structure. All that can be claimed, therefore, for this clas 
sification of the rocks of this region in reference to their erodibility 
is that it holds true only in the most general way and for large areas 
where there is likely to be more or less compensation between the vari- 
ous other factors, and it must be admitted that in such areas then 
would be exceptional localities where the determining factor was sonu 
other than the one made prominent in the definition of the group. The 
classification corresponds, however, to nature. There are such areas 
consisting mainly of gneiss, of schist, of limestone, and of shale a 
slate, upon which erosion has operated differently in degree or 
method. Erosion may have proceeded by physical processes aim 
as rapidly in a region of shale as by chemical ones in a region 
limestone. 
ROCK STRUCTURE. 
Having discussed the land form and its constituent rock inateri 
the structure of that material — that is, the rock structure — shou 
next be considered. 
General structural characteristics. — The general structural char 
acteristics of the region may be briefly summarized thus: All t 
rocks, including generally the underlying gneisses, lie in a success! 
of major and minor folds, corresponding approximately to the 
trend of the ranges. These structural characteristics are shown in two 
sections, PL V, ^1 and B (lines I and II on the map, PL I). Secti 
A crosses the recess in the Green Mountain Range, the Taconic Range 
the Rensselaer Plateau, and the lowland along the Hudson. Section 
begins 36 miles north of A, on the Green Mountain Range, in Manchl 
ter, crosses the Vermont Valley and the Taconic Range, but is resume 
L6 miles farther north in Wells, and ends in the lowland south 
Lake Champlain. These sections, of course, represent interpret* 
tions of structural observations. They bring out the anticlinoria 
character of the Green Mountain Range, the synclinorial structur 
of the Taconic Range, of its spurs and outliers, and of the Renl 
selaer Plateau, the complex structure of the Hoosic Valley west of th 
Green Mountain Range, and the synclinal structure of the Vermom 
Valley. They also show that in the Taconic region the valleys a 
generally limestone and the hills schist, and they show the horizont 
transition assumed by geologists from the harder rocks of Group ] 
to the slates and shales of Group IV at the west. The folds vary i 
character from close to open, erect to inclined, east or west. Careful 
studies of large areas show that these folds are not always continuous, 
but merge here and there, and that their axes are frequently inclined 
north or south. One of the very few points in which the inclination 
