508 
GEOLOGY OF THE FOURTH DISTRICT. 
Above the limestones last described, we meet with a “ black bituminous shale,” which, from 
position, seems to be the equivalent of the Marcellus shale of New-York,* and is the only 
representation of that rock, the Hamilton group and Genesee slate ; for we pass directly from 
this to the green shales and slaty sandstones of the Portage group or Waverly sandstone 
series of Ohio. In the examinations made in these rocks for several hundred feet upwards, 
no change from the Portage to Chemung groups could be identified, fossils for the most part 
being absent. I should not omit to state, however, that in passing beyond these greenish 
slaty rocks to a more micaceous and ferruginous yet friable sandstone, I found several shells 
which bear close analogy, if not absolute identity, with the Chemung species. But finding 
afterwards, in other parts of this sandstone, shells evidently belonging to carboniferous types, 
I was led to question the inference as to absolute identity.! Further investigations proved 
that this sandstone, in passing upwards, became interstratified with beds of limestone, and 
thin courses of oolitic limestone with fossils occurred in several places. These latter were 
not persistent, but in some places several inches thick and soon disappeared entirely, or left 
only a line of calcareous matter, marked by the presence of Producta. Still higher in this 
rock are some quarries, where a mass of limestone eleven feet thick is wrought for building 
stone. The lower part of this mass is a compact oolite, while the upper is rather coarsely 
crystalline with fragments of fossils. Below this, and separated by a course of sandstone 
of several feet in thickness, is another thick bed of limestone, and the whole is succeeded 
above by sandstone like that below. The height of these quarries above the black shale is 
four hundred and fifty-four feet; and the thickness of shales and sandstones between this 
point and the main limestone above, is fifty or sixty feet more.! 
These rocks I had denominated subcarboniferous; and although the fossils and the character 
of the intercalated beds of limestone indicate the commencement of the same era as the car¬ 
boniferous limestone, yet it requires that a limit should be fixed between what is to be strictly 
referred to the carboniferous period, and -older deposits. The grey sandstone here spoken of 
contained, in numerous localities, a large species of Productus, resembling P. hemispherica, a 
carboniferous fossil; while there seemed to be a gradual transition from rocks of the Chemung 
group to those above, indicating no cessation of deposition, and scarcely a change in litho¬ 
logical character, except the occurrence of thin beds of limestone.§ 
* Near New-Albany, this shale is one hundred and four feet thick ; “ in other situations it is only fifty feet thick.”— Second 
Ann. Rep. of Geological Survey of Indiana, p. 15. 
t The fossils referred to as similar to those of Chemung, are a species of Delthyris, a Strophomena, an Atrypa, and an Ino- 
oeramus. After examining a more extensive collection from the same situation, made by Mr. W. C. Redfield in Medina county, 
Ohio, I find that there are several species identical with those of'the Chemung group in New-York, and others which are entirely 
distinct. 
f The thickness given was furnished me from the surveys of road engineers in a letter from Dr. Clapp, of Sept. 2d, 1842. 
I find, in reference to the Report of Dr. Owen on the Geology of Indiana, that he has denominated the rocks here described, 
as well as the succeeding limestone, “ Subcarboniferous.” The limestone following is denominated in its different parts by Dr- 
Owen and Dr. Troost, as Oolitic, Pentremital and Archimedes limestone. 
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