MAHONING COUNTY. 791 
bench in the Mineral Ridge coal is the only representative of the block 
coal, while the upper bench, accumulated a little later, and only in 
the district where itisfound. That the blackband ore and its associated 
black shale were deposited in a lagoon or lake in the coal marsh, is 
proved by the great numbers of bivalve crustaceans, (Hstherva) found in 
it. Similar fossils usually accompany blackband ore and and are regard- 
ed as quite decisive as to its mode of formation. my 
The history of the deposit of the Mineral Ridge coal seems to have 
been something as follows: A broad and shallow basin was for a time 
occupied by a sheet of vegetation from which a stratum of peat of limited 
thickness was formed. This, from the nature of the vegetation, or the 
prevalent physical conditions, produced an open-burning or block coal. 
When the growth of peat was sufficient to produce from one to two feet of 
coal, the basin was flooded with water, and at least a part of it became a 
lake. In this lake a carbonaceous mud was slowly deposited, and when 
two feet in thickness, iron began to be precipitated with it in consider- 
able quantity. This formedthe stratum of blackbandore. Subsequently 
the lagoon was invaded and occupied by vegetation, and a thicker bed of 
peat than the first was accumulated over its surface; this second peat 
bed—probably from its more constant saturation or submersion in water 
—produced a more homogeneous and bituminous coal, the thicker upper 
bench. The causes which operated to produce the deposit of iron in 
this lagoon, were probably the shallowing of the water, its more com- 
plete evaporation, and thus the deposition of the iron which before 
flowed away in solution as a part of the freer drainage. In a similar 
manner we find the limestones of the Coal Measures, which were cer- 
tainly deposited in open bodies of water, generally capped with a 
stratum of iron ore; and we can plainly see that this was the last deposit 
in each of the water basins as it was disappearing. The accumulation 
of iron in our lakes and bogs at the present day is apparently produced 
in a similar way, although this is usually precipitated in the condition 
of limonite, the hydrated sesquioxide, becauge of the absence of carbone- 
ceous matter. 
BOWLDERS IN BLACKBAND AND COAL. 
Some years since I found in the blackband at the Weathersfield shaft 
an irregular, angular fragment of talcose slate. This had evidently 
been dropped into the carbonaceous mud, and, I have conjectured, from 
the roots of a floating tree where it had been entangled. No rock of this 
kind is found in place in Ohio, and the specimen is not rounded like a 
Drift pebble, it therefore seems probable that it was floated from the 
