INTRODUCTION. Skt 



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 front tliose then existing farther to tlio 

 west and southwest, that the lines of demarcation b«tween groups may not always correspond. 



