J02 Ulrich on Correlation of the Lower Silurian. 
the change is very sHght indeed, but in most cases the practiced 
eye has little difficulty in recognizing it. We have men, nat- 
uralists too, who are capable of appreciating very minute dis- 
tinctions, but when called upon to separate a lot of fossils, they 
cannot see the differences, and because they cannot see them, 
they reason that they do not exist. Such reasoning is, however, 
not good, as their inability to see is simply due to a lack of 
education and training in this particular department. Many 
minute and seemingly trivial variations exist, which are ordi- 
narily (especially by the stratigraphist) not at all taken into 
account. Yet they are of the utmost importance in stratigraph- 
icai determinations. Their value as markers of particular 
horizons must be manifest to all who can see that, if, (all things 
being equal,) large variations required a long time in their pro- 
duction, smaller deviations needed a correspondingly shorter 
time; hence, that the equivalences indicated by them are neces- 
sarily closer and more trustworthy. 
\Wq can not, of course, demand that the geologist who has 
made lithology and economic geology his principal studies, 
should also make himself familiar \vith the minute details of 
palaiontological subjects. No, geology has advanced too far 
to be mastered in detail by any single mind. We may, how- 
t'\er, reasonably expect that he will join forces with special 
students of palaeontology, for thus only are uniformly success- 
ful results possible. 
The subject of the study here contemplated, embraces three 
I'asins or large areas of exposures, besides several small patches 
along the Mississippi river in Illinois. 
In order that the reader may follow the evidence presented 
and form his own conclusions with the progress of the paper, 
I shall first consider each of the several series separately and 
without comment upon their equivalences, reserving the conclu- 
.sions until after all the facts relating to the questions at Issue 
"have been presented. 
As the series is moi"e complete and the exposures more exten- 
si\e in the Ohio valley than in either Tennessee or the north- 
west, they are taken wp first, leaving the consideration of those 
of central Tennessee, northern Illinois, Iowa, Wisconsin, Minn- 
eiiota, Canada and New York to follow in the order named. 
