51 



The basal formation of the Carboniferous Era in Indiana, as generally 

 elsewhere, is a bed of coarse-grained sandstone, known as the Mansfield 

 sandstone or "Millstone Grit." It has a total thickness of 150 feet 

 and forms tlie surface rock over a strip 2 to 22 miles in width, extending 

 from the northern part of Warren County in an east of south direction 

 to the Ohio River, a distance of ITo miles. In Martin and Orange 

 counties it occurs with an even, sliai-p grit, furnishing a most excellent 

 material for whetstones and grindstones. 



Above this sandstone are the I'roductive and Barren Coal Measiu-es, 

 which comprise T.oUO siiuare miles of tlie land surface of the State. 

 At the time of their deposition or formation the area which they cover, 

 as well as a large part of Illinois, was a great basin or depression, 

 but little above the level of the sea. and stirrotmded on every side ex- 

 cept the southwestern by the higher lands of the older formations. 

 By successive alternations of upheaval and subsidence— carried on 

 through thousands of years— this depression was at times an area of 

 the southwestern sea, again a fresh water lake, and then, for a period, a 

 vast swamp or marsh. ' AVhen raised high enotigh to form a marsh, the 

 luxuriant vegetation, above mentioned, sprang up from the ooze and mud 

 at its bottom, flourished for centuries, the newer growths spTinging from 

 between the fallen masses of the older, as in the peat bogs Of today, and 

 so formed a mighty mass of carbonaceous material. By subsidence, the 

 level of the marsh was, in time, lowered until it becan>e a lake into which 

 rivers from the surrotinding highlands flowed, bearing with them millions 

 of tons of clayey sediment and disintegrated qtiartz, the remgnns of the 

 older decayed rocks. This sediment was spread otit ovet .the mass of 

 submerged vegetation, compressing it into the hard, rhinefral coal; the 

 clayey sediment itself being in time compressed into vast beds of shale, 

 and the particles of quartz into sandstone. In some places a more pro- 

 longed subsidence took place, sinking the floor of the lake below the level 

 of the sea, and allowing the waters of the latter with their accompanying 

 forms of marine life to flow in. In time beds of limestone were then 

 formed over those of the shale or sandstone, but none of these cover an 

 extensive area or are of great thickness. 



After each subsidence, with its restiltiug beds of coal, shale and sand- 

 stone or limestone, had taken place, an upheaval followed. The floor of 

 sea or lake was again raised so near the surface that the semi-aquatic 



