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 it is found. 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, {Estheria) found in 

 it. Similar fossils usually accompany blackband ore and and are regard- 

 ed as quite decisive as to its mode of formation. 



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 formed the stratum of blackband ore. 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, because of the absence of carbona- 

 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 



