STEVENSON. oe 
Chemung and Catskill, 
The serious question now arises, how shall this great column, 
with a maximum thickness of more than 7,000 feet, be divided 
and what value shall be assigned to the divisions? Before under- 
taking to answer the question, let us recall the signification of 
the terms Catskill and Chemung as originally employed. 
Mather* in the annual reports as wellas in his final report used 
the term ‘‘Catskill Mountain Series’ to include all beds from the 
very highest rocks in the Catskill mountains down to the Cor- 
niferous, thus making it equivalent to ‘‘Formations VII, Ix, x, and 
xt of Prof. Rogers’ Report on the Geology of Pennsylvania for 
1838.” «Catskill group” was used first by Vanuxem in his 
final report, published in 1842,+ though he had previously fixed 
the lower limit, as may be seen by reference to the fourth report, 
where he takes the Montrose sandstone as representing the group. 
That rock rests directly on the Chemung group, whose upper 
limits are not so well defined in the fourth district of New York, 
though sufficiently so in the third. Its lower boundary appears 
to be distinct in both, so that the group consisted of the Chem- 
ung and Portage. But the Catskill group of authors is a varia- 
ble quantity. By some, the whole mass in the Catskills proper 
has been taken as belonging to the group; by others, the first red 
bed is taken as the base of the group, while others still see the 
beginning in the first fish bed. No one of these limitations 
suffices, for each is purely local in character and cannot be ap- 
plied over a great area. . 
Stratigraphically the two groups, as understood by Vanuxem, 
are separable without difficulty, The Catskill, including under 
that name, the Cherry Ridge shales of White and the Montrose 
sandstone of Vanuxem, is thoroughly persistent along the eastern 
outcrop from Greene county of New York to far beyond the 
James river in Virginia; its variations in thickness along this line 
are for the most part very similar to those of the upper and mid- 
dle divisions of the underlying Chemung. Westward and north- 
westward, however, the variations of the Catskill are unlike 
those of the Chemung. In southern Pennsylvania, the Cherry 
Ridge shales disappear within forty miles, while the Montrose 
sandstone thins out more slowly and does not disappear until Pay- 
)> 
ette county is reached; there, however, the whole mass, 3,700 
*Fourth Annual Report, p. 227. Geology of N. Y. Part I, p. 299. 
+Geology of New York. Part III, p. 186; 4th Ann. Rep. p. 381. 
