A Geological Section at Todd's Fork, 0. — Foerste. 415 
also of Huffman's quarry, south of Dayton. Here the quarry- 
men report the Clinton limestone to be thirteen feet thick al- 
though this may be slightly underestimated. Only a foot or 
so at the top is well supplied with fossils. Above this are 
about nine inches of a bluish stone containing a number of fos- 
sils distinct from anything in the Clinton below, many of them 
of small size. This is the Beavertown marl, at Huffman's 
quarry, sharply divided from the Clinton below, paleontologically 
and lithologically. In the upper fossiliferous (ferruginous) beds 
at Todd's fork however, they are blended together, the marl does 
not exist at all, and ssveral of the forms peculiar to the marl 
are here found commingled with abundant Clinton types. 
Orthoceras incept um and Raphistoma afflnis are the most com- 
mon of the marl fossils found here. Todd's fork thus offers a 
connecting bridge for these two divisions of Upper Silurian 
strata. In its lithological features it is also equivalent to the 
ferruginous Clinton of Kentucky. Some fossils are also com- 
mon to both, the most interesting of which is Ptilodicfi/a ex- 
pansa. The form with almost parallel sides is extremely rare, 
excf^pt at Todd's fork, where it is the only form seen. This 
other form is found abundant at the Soldiers' Home, and other 
quarries near Dayton. In it, the sides are more or less curved,, 
widening anteriorly and it cannot be distinguished from speci- 
mens of Pf. lanceolata collected in Gotland, an island near 
Sweden, The Kentucky Clinton is probably of slightly later 
age than the Clinton of Ohio. 
The Dayton limestone is a local variation of the base of the 
Niagara shales as recognized in Ohio, nnd consists of regularly 
bedded limestones of whitish color, passing above and at the 
edges gradually into the dolomitic limestones of the Niagara 
shales. Both limestones and shales are unfossiliferous at Todd'a 
fork, as is usually the case. At Huffman's quarry, however, 
near Dayton, various species of corals have at times been found, 
belonging to the genera Favosifes, Syringopora and Strombo- 
des; Stromhodes pygmwufi suggesting an age equivalent with 
the Niagara limestones of Michigan and Kentucky. The Wal- 
dron beds of Indiana correspond to the upper part of the Niag- 
ara shales, and contain a few forms introductory to the Guelph 
above. The West Union and Springfield rocks of Ohio geolo- 
gists often become merged with the Niagara shales and the 
