THE STORY OF OHIO'S ROCKS 



21 



in our Devonian seas but their environmental requirements were such that we find them in num- 

 bers or not at all. Devonian starfish and sea-urchins are known but were evidently scarce in 

 Ohio. 



Fig. 21 Devonian sea bottom 



The Devonian lands were beginning to look less desolate than those of the Silurian. There 

 were abundant algae in fresh water and sometimes their spore -cases were blown out to sea and 

 preserved. True land plants were beginning to appear, as witness tree -trunks as much as 5 

 feet in diameter, floated out to sea and preserved in our black shales, and fern-like plants pre- 

 served in the same way. 



Of Devonian land animals, we have a sure record only for amphibians, of which actual 

 skeletons have been found in Greenland. The scorpions of Silurian time undoubtedly had Devonian 

 descendants but no trace of them has been found nor, for that matter, of any Devonian land 

 invertebrates. 



MISSISSIPPIAN TIME: THE GREAT SANDY DELTAS 



At the beginning of the Mississippian period black shales continued to accumulate in 

 northern Ohio (fig. 22). Gradually, the black muds gave 

 way to silts and fine sands which sometimes became 

 coarser and graded into gravels which have consolidated 

 into conglomerates. In central and southern Ohio, sand 

 and silt were accumulating instead of black shales. 

 This was followed by a period of clear seas during which 

 limestones were deposited. Rocks deposited in the lat- 

 ter half of Mississippian time are not found in Ohio and 

 therefore we can only suppose that the state was dry 

 land in late Mississippian time. 



The Mississippian rocks of Ohio (fig. 23) have 

 yielded a variety of economic products. Oil and gas, 

 derived from fossil animals and plants, are found in 

 several Mississippian beds. Shales of the same age 

 are used in the manufacture of pottery and other cer- 

 amic products. Some Ohio Mississippian limestones 

 have a high calcium content and are used in the manu- 

 facture of lime. Mississippian sandstones were early 

 used for building in Ohio and are still extensively quar- 

 ried for that purpose. The Berea and Black Hand sand- 

 stones are used for foundry, filter, ferrosilicon, and 

 silicon and for silica refractories. They are washed 

 clean of impurities for glass and potter's flint. 



Fig. 22 Mississippian lands and seas 



