18 NEW YORK STATE MUSEUM 
plateau and Mohawk valley provinces where the hard limestone 
lies at an altitude of more than 1ooo feet, and directly overlies the 
soft shales of the valley whose altitude is only a few hundred feet. 
CATSKILL MOUNTAIN PROVINCE 
This is the most rugged of all provinces in the State and, next 
to the Adirondacks, contains the greatest elevations. Slide moun- 
tain (4205 feet) is the highest, while a number of points range 
from 3500 to over 4000 feet in altitude. 
The rocks are all of Devonic age and consist almost entirely of 
sandstones and conglomerates. Except for a slight westward un- 
dulation, these rocks are arranged in practically horizontal layers and 
show an aggregate thickness of several thousand feet (see figure 6). 
Lying under these Devonic rocks and outcropping at the very base 
of the mountains on the north and east, are various formations of 
Siluric age. 
The term ‘“ mountains ’”’ as applied to the Catskills requires some 
explanation. The more typical mountains of the world have been 
formed by folding or faulting of the strata, or by igneous activity, 
or by two or all of these causes combined. For example, in the 
development of the Appalachians both folding and faulting have 
played prominent parts, while in mountains like the Sierras or 
Adirondacks, folding, faulting, and igneous action have all been 
important. The Catskills, however, in which these typical moun- 
tain phenomena are wholly lacking, are to be properly placed in 
the category of what we may call “erosion mountains.” Moun- 
tains of the pure erosion type are due to an uplift of land high 
above sea level, followed by deep dissection of the elevated mass 
by the action of streams. The Catskills are only an easterly ex- 
tension of the plateau province where the rocks are more resistant 
and perhaps the elevation of the region was greater, so that the 
streams were able to cut deeper trenches while the harder rocks of 
the divides have so far prevented a general wearing down of the 
region. The Catskills furnish a remarkable example of a high 
plateau deeply dissected by numerous streams. The whole topog- 
raphy is very rugged, all being much like that of the highest por- 
tion of the Adirondacks around Mount Marcy (compare plates 
2 and 5). The Catskills, however, lacking the proper structural 
features, show practically no tendency to parallel arrangement of 
ridges or mountains as is so common in the Adirondacks. 
On the south the Catskill province almost grades into the folded 
region of the Appalachians, while on the west it gradually merges 
into the southwestern plateau. On the north the Helderberg 
