500 
GEOLOGY OF THE FOURTH DISTRICT. 
CHAPTER XXIII. 
On the identity of the Rock Formations of the Western States with those of New- 
York* 
Knowing that the different rocks and groups, as developed in the Fourth District, extended 
far to the westward, covering vast tracts of country, it was natural to feel a desire to examine 
them more minutely, and to draw my own conclusions from personal observation regarding 
their relative position. In the year 1841, I undertook a tour through the Western States, as 
far as the Mississippi river, having for my object the identification of the rocks and groups of 
New-York with those to which different names had been given by the Western Geologists. 
No extended attempt of this kind had been made from actual examination and comparison, so 
far as I know ; and the inferences from published reports, and the occurrence of certain fossils, 
had not proved satisfactory. The formations of the West, as described, did not correspond 
with the order as established in New-York; and the discrepancy could only be accounted for 
by supposing the thinning out of some important formations, or the occurrence of others not 
there existing. 
The similarity of some of the western formations with those of New-York, was first pointed 
out by Mr. Vanuxem, whose observations were published in the American Journal of Science 
and Arts in 1829. He identified the lower rocks of Ohio, Kentucky, and Tennessee, with the 
Trenton limestone, from the occurrence of many of the same genera and species of fossils 
common to both. I was referred by him to some localties which were important in settling 
the questions of identity or difference, and I am indebted to the same source for information 
of the existence of the Birdseye and Trenton limestones at Frankfort in Kentucky. 
Having, in New-York, adopted certain subdivisions or groups of the strata, which are 
strictly in the order of nature, it became a matter of much interest, to ascertain how far the 
same subdivisions would hold good in distant localities, where there was evidently great 
change in lithological characters. In employing geographical names for groups or individual 
rocks, it is desirable to know the locality of greatest development for the whole country; and 
when this is ascertained, the name should be adopted. But until the extent and comparative 
development of each rock is known, perfect local names cannot be prefixed; and as a step 
* This article is essentially extracted from a paper published in Transactions of the Association of American Geologists and 
Naturalists, Vol. I, page 267. 
