J. W. Powell—Types of Orographie Structure, 415 
theory of upheaval or collapse in the change of the relation of 
the several parts of these beds to the center of the earth. 
In treating of the structure of the mountains under consider- 
ation it is necessary to distinguish two great classes, viz: those 
composed of sedimentary strata, altered or unaltered, and those 
composed of extravasated material. 
MOUNTAINS COMPOSED OF SEDIMENTARY STRATA. 
I. Appalachian Structure. 
The structure of the Appalachian Mountains, with closely 
appressed folds and axial planes tipped back from the sea, the 
modifications of these folds by faults, and the primary and con- 
comitant forms of the mountains, have been clearly explained 
by the Messrs. Rogers and later writers, and have formed the 
asis of many discussions concerning geological dynamics. 
This Appalachian structure needs no further mention here, as 
It is a type of structure which so far has not been found in the 
region described above, and should it be found hereafter it will 
simply be an exceptional type to those known to prevail. 
Il. Simple Anticlinal Structure. : 
Mountains or short ranges carved from simple anticlinals are 
ai 
sometimes found, though this type of structure 1s not a pre 
streams running down their sides. Sometimes such mountains 
are severed ) 
obliquely through them; the rivers themselves having their 
~ources In regions far away and passing through the mountains 
it their courses to the sea. In Northeastern Colorado a short 
<a ae ii ich the Yampa River 
parts by a cafion, through which the Yampa 
moe. The axis of the mountain has a north and south direction. 
‘gure 1 
South directio i Figure 2 is a sec- 
tion th n, along the axis of upheaval. Fig 
