INTRODUCTION. 
51 
limestone have had a line of greater accumulation; and that it is 
demonstrable, from the combined investigations of geologists, that this 
line was along the course of the Appalachian range. In the second 
place, all the observations carried on through New-York, Ohio, Indiana, 
Michigan, Wisconsin, Illinois, Iowa, and Missouri, show a thinning of 
these sediments in a westerly direction, until, in the Mississippi valley, 
they have greatly attenuated or entirely disappeared. 
Following the Chemung group, we have in New-York the shales and 
sandstones of the Catskill mountains, forming in their greatest expan¬ 
sion a mass of at least three thousand feet in thickness. To the north¬ 
east this formation has not been recognized as a distinct group, though 
we infer that it may be included in the great mass constituting the 
section of seven thousand feet of strata shown by Sir William Logan 
to exist between the limestones below and the Coal measures above*. 
In New-York this group has its greatest expansion just to the west 
of the metamorphic belt, on the west side of the Hudson river; raising 
its summit (including the conglomerate) in the Catskill mountains to 
the height of 3800 feet above tide water. The group is composed 
of red and greenish or olive shales, and shaly sandstones, with some 
gray and mottled sandstones and conglomerates, the latter forming 
heavy masses at the summit of the formation. 
This group presents a very interesting topographical feature, and 
one of even greater interest in the dynamics of geology; for we have 
here mountains of nearly horizontal sedimentary and almost entirely 
unaltered strata, consisting mostly of a single group, attaining an ele¬ 
vation rivalling that of the disturbed metamorphic belt upon the east, 
where the highest points are rarely more than four thousand feet 
above the level of the sea. 
* It seems to me not improbable that the earlier appearance of terrestrial vegetation in the northeast, or 
at least the greater amount of such vegetation, during the earlier part of the Devonian period, may have 
resulted from, or have been accompanied by, conditions so different *from those then existing farther to the 
west and southwest, that the lines of demarcation between groups may not always correspond. 
