The Maquoketa Shales. — James. 345 
cur. There is no vertical exposure but the slope is undoubt- 
edly over the upper beds of Lower Silurian age. 
On the opposite side of the railroad from the Post Office is 
a hill, probably two hundred feet high. On top of this has 
been opened a small quarry with a face of about fifteen feet, 
the limestone in courses of from four to six inches, and with 
considerable quantities of chert in places. Among the debris 
here I found a specimen of Favosltes (jothlandica^ indicating 
a Niagara horizon. 
The slope from the bottom of this hill to within about forty 
or fifty feet of the top is gradual. From this point to the top 
it becomes quite abrupt, and this abrupt portion probably rep- 
resents the Niagara limestone. The shales and limestones of 
the Lower Silurian are easily broken down ; but the heavier 
courses of the Niagara resist atmospheric action and are not 
so easily affected. The change from a gradual to an abrupt 
slope is noticeable on all the hills in the vicinity, so that the 
Niagara doubtless caps all the high ground. Both the Niagara 
and the Maquoketa disappear toward the east, and as Dubuque 
is approached the Galena limestone comes to the surface. 
In geologic nomenclature it is occasionally expedient to give 
to exposures of rocks in different parts of the country distin- 
guishing names. It has sometimes been done because the 
facts at hand have not permitted a correlation of the rocks of 
two or more widely separated sections of the country. Thus 
the names Hudson river, Lorraine, Nashville, and Cincinnati 
have been given to groups of rocks in different parts of the coun- 
try, though all of them are referable to the same series. The 
term Lower Magnesian was applied to a series of rocks in the 
northwest, before it was known that these were of Calciferous 
age. Le Claire limestone was given to another series because 
it was thought to be distinct from the Niagara. Similarly Maquo- 
keta was applied to a formation because it was supposed to 
represent a distinct epoch of the Cincinnati group. 
There are certain rules that must govern the coining of new 
names for formations in new localities. The series of rocks 
must represent a distinct period in geological time and one 
that has not been previously named and described. They 
must be shown to be distinct from previously named series by 
a difference in lithologic combined with pakeontologic features. 
Or else by showing that the newly proposed group or formation 
