Cincinnati Silurian Island. — Miller. 83 
Surrounding the Lexington limestone area comes the Cin- 
cinnati blue limestone area, about 12,000 square miles in ex- 
tent. The thickness of the formation is from 600 to 800 feet. 
Outliers of Niagara and Corniferous limestone lie far within 
the boundaries of this region. One of these, constituting 
Jeptha knobs in Shelby county, Kentucky, on the western 
limb of the anticline, lies approximately half way between the 
internal and external boundaries. The hight of the Cornif- 
erous upon the top of these knobs is between 1,200 and 1,300 
feet above sea level — high enough to peep over the arch at its 
highest point. This proves that the formations up to and in- 
cluding the Corniferous went over the whole combined Lex- 
ington and Cincinnati areas. 
A little nearer the margin of the Cincinnati area, hvX still 
well up toward the crest of the arch, where it begins to pitch 
southward under the Devonian and lower Carboniferous, is 
Burdett's knob, in Garrard county, Kentucky. This is also a 
faulted outlier. It rises to a hight of 1,090 feet above tide and 
preserves the remainder of the Devonian (the Black shale) 
with a thin capping of Lower Carboniferous (Waverly). By 
the same reasoning it seems impossible to escape the con- 
clusion, that the Lower Carboniferous went over the area in 
question; and if the Lower Carboniferous why not the Upper? 
Outcropping all around the margins of both the Eastern and 
Western coal fields is the great basal conglomerate. It rises 
into great cliffs, which for the Eastern field reach an altitude of 
1,400 feet where they overlook the central blue grass counties 
of Kentucky. A restoration of the air lines of this formation 
easily carries it over this region, and over the very summit 
of the Lexington dome it one time must have gone. 
We may form some idea of how high this would have 
made our arch had it not been denuded. Green River knob, 
in Pulaski county, Kentucky, on the sunmiit of the arch where 
is sinks to its lowest level before rising to the Tennessee 
maximum, is 1,800 feet above the sea. It preserves on its 
summit an outlier of Kaskaskia sandstone just beneath the 
Coal Measures. As has been noted by Prof. Shaler, from the 
top of this knob on any clear day may be seen the outliers of 
the Eastern and Western coal fields, here some 85 miles apart. 
Undoubtedlv the Coal Measures once extended over southern 
