92 The American Geologist. August. 1903. 
The Middle Shale. — There were no notable changes in the 
outline- of land and water during- the middle portion of the 
great shale period, when, for the most part, the beds deposited 
were of a lighter color. This part of the mass is strictly lim- 
ited to the Appalachian gulf. Consequently, on the Indiana side 
of the arch and beyond that line, the eighty or one hundred feet 
of black shale must be taken to represent the whole of the huge 
mass found in ( Mho. Of the rapid thinning of this part of the 
Ohio shale as it approaches the line of the arch there can be no 
doubt — a proof, if any were needed, of the relative levels at that 
day. 
The Upper Shale. — The upper part of the Ohio shale gives 
evidence of a wide extension of the sea beyond its previous 
limits. The southern border of the Appalachian gulf, which 
had been for ages narrowing and restricting the water-area, 
suddenly widens out, allowing the Upper Devonian sea to 
transgress its previous boundaries in that direction. Over the 
edge of the older strata in Kentucky, Tennessee and Alabama 
there lies a black shale of small thickness, but of great persist- 
ence — the only representative of the Devonian in that region. 
It can be traced as far south as any of the older strata and with 
them it disappears under the Cretaceous and Tertiary beds of 
the gulf states. It is involved with them in the folds of the 
Appalachian range in the south, but it is not seen as a black 
shale in their more northerly extension in Pennsylvania. 
In no other direction can we detect any marked change in 
the outline of the Appalachian gulf in Upper Devonian days. 
Nor is it possible to determine to what extent the waters over- 
flowed the Cincinnati-Xashville promontory, though there is lit- 
tle room to doubt that it disappeared to a very great extent be- 
neath the waters, leaving perhaps two islands as monuments of 
its existence. The overlap of the shale in southern Ohio was the 
prophetic indication of this greater transgression in the follow- 
ing period, when the geography of the region was brought 
back for a time almost to its primeval condition in early Silur- 
ian days. 
We are thus presented with a second wide palaeozoic ocean, 
extending from the estuary of Xew York southwestward to an 
unknown distance, and possibly re-opening communication 
with a southern ocean, if any such then existed, by a wide chan- 
