338 The American Geologist. June, i89o 
the resemblance." A similar shale wais thrown out of a shaft some 
two miles west of Dubuque, which contained broken fossils, and in the 
same vicinity Nucula or Tellinomya were found. "The entire thickness 
is probably less than seventy-five feet, and aj^parently but little more 
than sixty feet." 
"1q consequence of the easterly direction of the river, (Mississippi), 
the shales of the Hudson River group continue above the water level, 
and appear at their full development as low down as 8abula ; where 
the clift'at Savannah, on the opposite side of the river, gives a section 
of some eighty or ninety feet. At this place the calcareous bands have 
increased in thickness and frequency ; and the whole mass has much 
the same appearance as at Cincinnati, Ohio, and Madison, Indiana. 
Among the fossils occurring here are Orthis occidentalis , 0. testudinaria, 
Strophomena alternata, S. Jilitexta, and others which do not occur in the 
exposures of these shales farther to the north." "The shales of this 
group finally disappear beneath the river before reaching Lyons, at 
which point the Niagara limestone comes to the level of the river. We 
shall pi'obably be able hereafter to find some sections of this group far- 
ther to the Northwest, which may prove its character and thickness. 
All the facts at present known regarding it, show that it becomes grad- 
ually thinner in that direction ; and we infer from the exposures ob- 
served, that it does not exceed seventy-five feet in thickness (and is 
probably less than that) on the branches of the Little Makoqueta Creek.* 
On the Ohio river at Cincinnati it is more than five hundred feet in 
thickness, while the geological report of Missouri gives to this group one 
hundred and twenty feet. The great development which it attains in 
eastern localities, compared with these observations, shows that there 
is a constant diminution to the westward ; and we may expect to find 
its greatest tenuity or absolute dissappearance from thinning out, 
somewhere about the head waters of the western branches of Turkey 
River." 
In 1861 professor Hall publishecP descriptions of some new 
species of fossils from the Potsdam, Trenton, Hudson River 
and Niagara groups. Among these species vi^ere the following 
from shales overlying the Galena limestone on Little Maquo- 
keta river, in Iowa. Graptolithus peosta, Pleurotomaria 
semile, Cyrtoceras whitneyi., Orthoceras greg avium (after- 
ward changed to 0. sociale), and Calym^ene mammilata. 
In 1862 professor Hall described' the series which he had 
previously referred to the Hudson River group, under the 
name of "Green and Blue shales and limestones." Among 
these rocks he places the beds found on the Little Maquoketa 
river in Iowa, and after quoting the section as given in the 
Iowa report,^ gives the details of another section as found near 
Scales mound in Illinois just south of the Wisconsin line 
This section is as follows : 
*The name is thus spelled by Professor Hall, 
^Rept. of Supt. of the Geol. Survev (of Wisconsin), Madison, 1861, 
pp. 52. 
'Geol. Survey of Wisconsin, vol. I. pp. 47-55. 
**See ante p. 338. 
