92 The American Geologist. August, 1904. 
feet and the interval between the base of the HchcrtcUa insciilp- 
ta layer and the Clinton is 96 feet. Along the railroad at Mad- 
ison, the Locke level measurements for the thickness of the 
Richmond are 170 feet, and the interval between the base of the 
H chertella insculpta layer and the Clinton is 85 feet — both along- 
the Canaan road two miles northeast of Madison, and along 
the Hanging Rock road at the northern edge of the city. Mak- 
ing full allowance for errors due to Locke level readings and to 
dip, a local increase rather than a diminution of the interval 
between the base of the H. insculpta layer and the base of the 
Waynesville bed is suggested. South of Madison, HcbcrteUa 
insculpta is unknown. 
The interval between the base of the HchcrtcUa insculpta 
layer and the top of the Whitewater or the base of the iMadison 
bed, on the contrary, diminishes rapidly southward. Half a 
mile southwest of Cedar Grove_, on the road to South Gate, this 
interval is 140 feet. Dinorthis suhquadrata has a range extend- 
ing from 10 feet above the base of the HcbcrteUa insculpta 
layer to 150 feet above the same. The exposure was divided 
into vertical sections of 10 feet, and in each of these Dinorthis 
suhquadrata was found. The rubble limestone overlying this 
Dinorthis section is referred to the Madison bed. At A'er- 
sailles, the interval between the base of the HcbcrteUa insculpta 
layer and the base of the Madison bed layer is 61 feet. The 
base of the lowest massive Tctradiuin layer is taken as the base 
of the Madison, although Coluninaria alvcolata is found oc- 
casionally about a foot beneath this Tetradium layer. Two 
miles southeast of Belleview, Hehertella insculpta occurs 44 
feet below a Coluninaria alvcolata layer. At Madison, along 
the Hanging Rock road, a few poor specimens of HchcrtcUa 
insculpta were found loose about 31 feet below- the lowest Col- 
uninaria alvcolata bed which here forms the base of the Madi- 
son. This is probably the true horizon for HcbcrteUa insculp- 
ta, since the lowest specimens of Dinorthis suhquadrata at this 
locality occur about 28 feet below the lowest Coluninaria layer, 
and since in that case the interval between the HchcrtcUa in- 
sculpta layer and the Clinton would be 85 feet, an interval 
known to occur along the Canaan road two miles northeast of 
Madison. 
